fix(coding-agent): hint marketplace-qualified spec when bare plugin install fails

[yazzang-homelab]

What

gjc plugin install <bare-name> now prints the marketplace-qualified spec that would work when the npm fallback fails.

A bare spec (no @marketplace) is classified as npm by design (classifyInstallTarget rule 3). That rule is unchanged — only the failure path gained a hint, plus a new zero-dep src/cli/marketplace-hint.ts holding the two pure helpers.

Why

gjc plugin discover lists bare plugin names. Copying one into gjc plugin install resolves against npm, fails, and prints only install_failed, which names neither the cause nor the working spec.

Observed on a clean isolated HOME against the official Anthropic marketplace:

$ gjc plugin marketplace add anthropics/claude-plugins-official
✔ Added marketplace: anthropics/claude-plugins-official
$ gjc plugin discover | grep claude-md
  claude-md-management
$ gjc plugin install claude-md-management
✘ Failed to install plugin (install_failed)          # dead end
$ gjc plugin install claude-md-management@claude-plugins-official
✔ Installed claude-md-management from claude-plugins-official (1.0.0)

After the change the first form ends with Try: gjc plugin install claude-md-management@claude-plugins-official.

Only marketplace names are echoed, and only for a bare spec (no @scope, version, or path separator), so the install-failure path keeps its rule of never printing a spec or cause that can carry credentials or a home path.

Testing

• bun test packages/coding-agent/test/marketplace/cli.test.ts — 14 pass (4 new cases: bare-name shape gate, multi-marketplace hit ordering, absent name, unreadable catalog skipped).
• E2E from source in an isolated HOME with the official marketplace registered: bare name prints the qualified hint; definitely-not-a-plugin-xyz and @scope/nope print no hint.
• bunx biome check clean on the touched files.
• bun run check:types NOT completed: tsc (typescript-go) aborts with fatal error: runtime: name offset out of range on this machine at dev baseline too, i.e. before this change. Needs CI to cover the typecheck.

GJC verdict

gajae.pr-review-verdict.v1 needs-human sha256:ac7a532debd4f74bb60d4e7f7bc347ac59e9c8af reviewer:human evidence:bun test packages/coding-agent/test/marketplace/cli.test.ts (14 pass) + isolated-HOME e2e above

---

• Target branch is dev
• bun check passes — biome clean; tsgo typecheck crashes at baseline on this host, left to CI
• Tested locally
• CHANGELOG updated (if user-facing)
• Verdict above matches the exact PR head, not an earlier commit

[yazzang-homelab]

Rebased onto current dev (11e48d5bc, #3901) — the previous head was CONFLICTING/DIRTY on the CHANGELOG ## [Unreleased] section. New head 39a668756; rebase was CHANGELOG-only, no code merges, and the diff is otherwise unchanged.

Re-verified on the new base: bun test packages/coding-agent/test/marketplace/cli.test.ts → 14 pass, 0 fail.