feat: mirror opencode skills into .cursor/skills/ for Cursor agent discovery

[justin-carper]

Supersedes #78. Wayne's original commit (cee7d16) is carried over unmodified,
with authorship preserved; the three commits on top resolve the review feedback
left on that PR.

Summary

Mirrors opencode's resolved skills into <cwd>/.cursor/skills/ so the Cursor
agent (and Cursor sub-agents) discover and load them natively via the project
settings layer. Skills are discovered from the standard opencode locations plus
config.skills.paths, filtered through the live permission config, and
materialised as a git-ignored mirror stamped with a generated: opencode-cursor
sentinel. Only sentinel-bearing files are ever overwritten or deleted, so
user-owned .cursor/skills/<id>/ directories are safe. An <available_skills>
catalogue is appended to the generated system rule, and the mirror re-syncs each
turn behind a hash gate. Full detail in #78.

Review fixes on top

de89036 — fix(skills): follow symlinked skill dirs and files

Dirent.isDirectory() returns false for a symlink pointing at a directory, so
a skill linked in from a shared checkout was dropped from the mirror with no
warning. Symlinked supporting files were lost the same way in collectFiles and
copyTree. Entries are now classified through the link target via a shared
entryKind helper, with a resolved-path visited set guarding the cycles that
following links admits. Broken symlinks and links to non-regular files are still
rejected. Six tests added, three of which were confirmed red before the fix.

053e4eb — docs(skills): correct deny scope, note plugin-skill gap

skills.include is checked before permissions resolve, so it re-admits a denied
skill by design. SECURITY.md read as though deny were absolute — corrected,
since that is the file someone reads when deciding whether deny is a hard
guarantee.

Skills bundled inside opencode plugin packages land in the package cache
(~/.cache/opencode/packages/<pkg>/node_modules/<pkg>/skills/), which is not a
scanned location, and @opencode-ai/sdk exposes no skills API to resolve them.
Now documented in the README limitations list with the config.skills.paths
workaround, so a user does not quietly receive a subset of their skills.

fb44fb6 — style(plugin): normalize indentation in skills bridge blocks

index.ts block re-tabbed to the surrounding level, de-dented closing braces
corrected, and tabs in the otherwise space-indented cursor-tools.ts delegate
call converted to spaces.

Verification

npm run typecheck && npm test && npm run build

typecheck: clean
Test Files  32 passed (32)
     Tests  409 passed (409)
build:     ESM + DTS success

Limitations

• config.skills.urls (HTTP catalogs) are not mirrored.
• Skills bundled inside opencode plugin packages are not discovered.
• cursor_cloud_agent targets a remote repo and does not inherit skills.
• cursor_delegate with a non-session cwd does not mirror into that cwd, but
  passes settingSources: ["project"] so a pre-existing mirror there loads.