fix(cli): accept source prefixes in the wizard package prompt

[mrrobertkent]

Problem

The interactive wizard rejects the source prefixes documented for skilld add, with a stack trace rather than a message:

◇  Enter package names (space or comma-separated)
│  npm:unlighthouse
│
◑  npm:unlighthouse: npm registryError: Invalid package name: npm:unlighthouse
    at getCacheDir (dist/_chunks/prepare.mjs:15:40)
    at resolvePkgDir (dist/_chunks/prepare.mjs:30:29)
    at getShippedSkills (dist/_chunks/prepare.mjs:53:18)
    at handleShippedSkills (dist/_chunks/skill-installer2.mjs:14:24)
    at npmResolver (dist/_chunks/sync.mjs:34:20)

■  Canceled

add normalizes its inputs:

// src/commands/sync/add.ts
const items = rawInputs.map(parseSkillInput)

The wizard does not:

// src/cli.ts
selected = input.split(COMMA_OR_WHITESPACE_RE).map(s => s.trim()).filter(Boolean)

So npm:unlighthouse travels as a literal package name until getCacheDir rejects it, and the throw takes down the whole run.

Change

Normalize the same way add does, at both manual-entry prompts. npm: prefixed and bare names resolve; other sources report where they belong instead of failing later:

gh:owner/repo is not an npm package. Install it with `skilld add gh:owner/repo`.

Second, resolvePkgDir no longer lets the validation error escape. It probes for an optional cache hit, so a malformed name is a miss. Traversal is still rejected, it just no longer aborts a sync from a read path. getShippedSkills is the only caller that could reach it with unvalidated input, and it already treats "not found" as empty.

Testing

test/unit/pkg-dir-probe.test.ts covers the probe returning null for npm:vue, gh:owner/repo, and ../escape rather than throwing, and getShippedSkills reporting none.

Normalization verified against the wizard's own splitting:

npm:unlighthouse           -> ["unlighthouse"]
unlighthouse               -> ["unlighthouse"]
vue npm:nuxt pinia         -> ["vue","nuxt","pinia"]
gh:harlan-zw/unlighthouse  -> rejected with guidance
crate:serde                -> rejected with guidance

Test Files  57 passed (57)
     Tests  908 passed (908)

pnpm typecheck clean.

Docs

README notes that the same prefixes work in the wizard prompt, and the placeholder now shows one (vue npm:nuxt pinia).

Out of scope

resolvePkgDir('', cwd, v) returns <cwd>/node_modules, because join(cwd, 'node_modules', '') exists. No caller passes an empty name, so I left it alone rather than widen this PR.

[mrrobertkent]

Closing in favour of a clean branch.