docs(rfc): propose the multi-provider vp doc command

Vite+ had a prototype vp doc command that bundled VitePress; it was
removed before the RC release. This RFC brings the command back as a
public surface without bundling any documentation generator: the
project selects a provider, Vite+ detects it from declared
dependencies, translates dev/build/preview, and executes the
project-installed tool. Four built-in providers: VitePress 2, Vocs,
Starlight, and Ox Content, each supported from its first release that
runs on Vite 8. Covers vp doc init and info, the defaultPackage doc
entry with the workspace picker and listing, opt-in build caching
through run.tasks, native-config validation for integration providers,
and the security invariants. rfcs/cwd-flag.md gains the doc entry of
the defaultPackage object form.

Author: fengmk2

rfcs/cwd-flag.md

@@ -5,7 +5,7 @@
 Add a global `-C <dir>` flag to vp, then use it to fix the app-command experience in monorepos. Everything is additive and backward compatible:
 
 1. **`-C <dir>` global flag** (the feature): for every vp command, `vp -C <dir> <cmd>` behaves exactly like `cd <dir> && vp <cmd>`, following the `git -C` / `make -C` convention: the first-class "run there" form vp lacks. Positional semantics stay untouched: `vp dev <path>` keeps upstream Vite semantics (`root` only), and `vp pack` positionals stay tsdown entries.
-2. **App-command UX built on `-C`**: running `vp dev` / `vp build` / `vp preview` / `vp pack` bare at a workspace root elicits the missing target instead of silently running against the root: an interactive fuzzy package picker in a TTY, a `defaultPackage` config for repos with one blessed target, and a clear listing plus exit 1 when non-interactive. All three are defined as an implicit `-C <dir>`.
+2. **App-command UX built on `-C`**: running `vp dev` / `vp build` / `vp preview` / `vp pack` bare at a workspace root elicits the missing target instead of silently running against the root: an interactive fuzzy package picker in a TTY, a `defaultPackage` config for repos with one blessed target, and a clear listing plus exit 1 when non-interactive. All three are defined as an implicit `-C <dir>`. `vp doc` joins the same model through [doc-command.md](./doc-command.md).
 
 In the common flows users never type `-C`: bare `vp dev` at the root goes through the picker or `defaultPackage`, and inside a package it runs there as today. `-C` is the explicit, teachable form underneath. The commands stay singular: `vp dev` still starts exactly one Vite dev server in one directory. Running a task across many packages at once remains the job of `vp run` (`-r`, `--filter`).
 
@@ -257,6 +257,8 @@ An app command invocation is **bare** when it has no `-C` and no positional targ
 
 "Workspace root" means the current directory's package is the workspace root package, as determined by `vt_workspace::find_workspace_root` (already called on every invocation in `packages/cli/binding/src/cli/mod.rs`).
 
+`vp doc` ([doc-command.md](./doc-command.md)) follows the same resolution order with one substitution: the picker in step 4 and the listing in step 5 show only workspace packages that declare a documentation provider marker. The `doc` entry of the `defaultPackage` object covers step 3, and the redirect covers every `vp doc` subcommand, `init` and `info` included. `vp doc` has no directory positional, so step 2 never applies; every argument after `dev`, `build`, or `preview` belongs to the tool. Provider detection itself never scans beyond the nearest manifest, so the doc picker and listing remain the only workspace enumeration.
+
 ### Equivalence invariant
 
 For every vp command:
@@ -309,12 +311,13 @@ export default defineConfig({
 });
 ```
 
-- Type: `string` (one directory for all four commands) or a per-command object (`{ dev: './apps/web', pack: './packages/ui' }`, added on review demand). A command absent from the object falls through to the picker/listing resolution.
+- Type: `string` (one directory for the four app commands; never `doc`) or a per-command object (`{ dev: './apps/web', doc: './packages/docs' }`, added on review demand). The object gains a `doc` key with `vp doc`, and `vp doc` participates only through the object form. A command absent from the object falls through to the picker/listing resolution.
 - Consulted when a bare app command runs in the directory containing the root config: a workspace root, or a non-workspace repo root. The non-workspace shape has no package list, so `defaultPackage` is the only mechanism that covers it. An explicit `-C` always wins.
 - A missing directory errors: `defaultPackage points to a missing directory: ./frontend`.
 - Read via static extraction (`vp_static_config` + the loader in `packages/cli/binding/src/cli/handler.rs`), like `run` config. At a non-workspace root there is no install to execute the config, so the file must work unexecuted: a plain default-export object with a static string value.
 - Only an explicitly declared `defaultPackage` changes behavior. A declared but non-static value (e.g. `process.env.DIR`) errors; a config that is unanalyzable or hides fields behind a spread is treated as not declaring the key and falls through to the picker or current-dir resolution, so an exotic config can never break unrelated bare commands.
 - Consulted only at the invocation root (a workspace root, a standalone package root, or a directory with no `package.json` ancestry). Below a workspace root the current directory already identifies the target, so a member package's own config never redirects.
+- `doc` is a command key for `vp doc` ([doc-command.md](./doc-command.md)). The string form covers the four app commands only, never `doc`: the blessed app package and the documentation package are usually different packages, so `doc` participates only through an explicit object entry.
 
 ## Decisions
 
@@ -332,7 +335,7 @@ Below the root the cwd already identifies the project, so prompting would be noi
 
 ### Elicitation scope
 
-`-C` is global and works with every command. The elicitation behaviors (picker, `defaultPackage`, root error) apply only to the single-target app commands, because only they are ambiguous at the root. Tree-scoped commands (`test`, `lint`, `fmt`, `check`) mean "the whole repo" there, which is their desired behavior. Workspace-state commands (`install`, `add`, `outdated`, ...) have the root as their natural home. Orchestrators (`run`, `exec`) own their selection models and remain the way to run one task across many packages. A future command joins the elicitation set exactly when its subject is one package directory.
+`-C` is global and works with every command. The elicitation behaviors (picker, `defaultPackage`, root error) apply only to the single-target app commands, because only they are ambiguous at the root. Tree-scoped commands (`test`, `lint`, `fmt`, `check`) mean "the whole repo" there, which is their desired behavior. Workspace-state commands (`install`, `add`, `outdated`, ...) have the root as their natural home. Orchestrators (`run`, `exec`) own their selection models and remain the way to run one task across many packages. A future command joins the elicitation set exactly when its subject is one package directory. `vp doc` is the first such addition: its subject is the documentation package, its config default is the `doc` entry of `defaultPackage`, and its picker filters to provider-marker packages ([doc-command.md](./doc-command.md)).
 
 ## Implementation Architecture