Add read-only installed skill checks

[nahiyankhan]

Category: new-feature
User Impact: Users can detect installed ghost instructions that differ from the CLI's bundled version.

Problem: An installed skill can fall out of sync after a CLI upgrade, with no command to compare its managed files.

Solution: Add ghost skill check using install's destination resolution. Report missing, changed, and extra reference files without writing, and suggest a reinstall command.

Compatibility: Exit 0 means matching files, 1 means differences, and 2 means invalid arguments. Checking does not establish which instructions an active session loaded; review local edits before reinstalling.

Scope: One of five independent PRs split from the original #291. This branch targets main and contains only this concern, its tests, and its documentation.

Related: Enforce resolved material access boundaries, Preserve guidance meaning in CLI output, Report incomplete guidance loading, Fix and validate rendered guidance examples.

Validation:

• pnpm run quality:all: passed on this standalone branch, including build, package/release checks, tests (246 passed), workspace builds, and package validations.
• Pre-commit checks and git diff origin/main...HEAD --check: passed.
• Fresh dependency installation hit registry checksum errors. Validation used an isolated copy of the existing checkout's dependencies; the lockfile is unchanged.

Changeset: Existing minor changeset retained for the new command.

ghost Review: Review packet assembled against apps/docs/.ghost. This package does not bind the changed CLI/test/docs files, so the packet is coverage context, not a passing brand review. Used the current ghost review --package apps/docs/.ghost --base origin/main --format json and inspected ghost manifest; the older ghost check, --include-memory, and dump:cli-help workflows are unavailable in this checkout.

File changes (9 files)

File	Purpose
.changeset/check-installed-skill.md	Record the new read-only command.
README.md	Document checking after upgrades and list the command.
packages/ghost/README.md	Document checking after upgrades and list the command.
packages/ghost/src/commands/command-discovery.ts	Expose checking in command discovery.
packages/ghost/src/commands/skill-check.ts	Compare managed files and format differences without writes.
packages/ghost/src/commands/skill-command.ts	Add the check action with shared destination resolution and argument validation.
packages/ghost/src/skill-bundle/SKILL.md	Explain post-upgrade checks and their runtime limits.
packages/ghost/src/skill-bundle/references/schema.md	Document destinations, exit codes, and read-only behavior.
packages/ghost/test/skill-check.test.ts	Cover fresh and stale installs, destinations, read-only behavior, symlinks, and I/O failures.

Screenshots/Demos: N/A: no visual surface changes.