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

[WayneSimpson]

Summary

This PR adds a skills bridge that mirrors opencode's resolved skills into <cwd>/.cursor/skills/ so the Cursor agent (and Cursor sub-agents) can discover and load them natively via the project settings layer.

What it does

• Discovers skills from all standard opencode locations: project .opencode/skills/, .claude/skills/, .agents/skills/ (walked up to the git worktree root), global ~/.config/opencode/skills/, plus config.skills.paths directories (lowest priority, first-wins on duplicate ids).
• Filters through opencode's live permission config (allow/deny/ask). ask-permissioned skills are withheld (the ask prompt can't cross the Cursor boundary). Manual skills: { include, exclude } override is supported with wildcard patterns.
• Materialises as a git-ignored mirror with a generated: opencode-cursor sentinel in each SKILL.md's frontmatter. User-owned skill directories (no sentinel) are never overwritten. Supporting files are copied alongside, with per-file 1MB and total 10MB caps.
• Catalogues skills in an <available_skills> section appended to the generated system rule, so the Cursor agent knows what's available and can load skills on demand.
• Re-syncs each turn via the chat.params hook (hash-gated, includes all file mtimes + sizes). The current catalogue is always forwarded per turn, including an empty string when all skills are removed (clearing the catalogue from the rule).
• Disposes cleanly — removeSkillMirror and removeSystemRule delete only sentinel-bearing files on plugin dispose.

New files

File	Purpose
src/plugin/skill-discovery.ts	Filesystem walk, frontmatter parsing, permission filtering, path expansion, skillSetHash
src/provider/skill-mirror.ts	Materialisation, sentinel stamping, git-ignore, stale-dir pruning, size caps, dispose, catalogue builder
scripts/verify-skill-mirror.mjs	Standalone end-to-end verification script
test/skill-discovery.test.ts	25 tests — discovery, filtering, precedence, wildcards, hash
test/skill-mirror.test.ts	14 tests — materialisation, sentinel, idempotency, pruning, size limits, dispose
test/plugin-skill-mirror.test.ts	10 tests — config hook integration, chat.params re-sync, dispose, forwardSkills opt-out

Updated files

• src/plugin/index.ts — forwardSkills option, config-hook materialisation, chat.params live re-sync, dispose cleanup
• src/provider/language-model.ts — dynamic + static catalogue resolution (no self-provisioning — respects forwardSkills: false)
• src/provider/system-rule.ts — writeSystemRule/resolveSystemDelivery accept optional skillsCatalogue
• src/provider/delegate.ts + src/plugin/cursor-tools.ts — settingSources: ["project"] for delegate
• src/provider/index.ts — skillsCatalogue in provider options
• README.md — Skills section, options table, limitations
• CHANGELOG.md — [Unreleased] entry
• SECURITY.md — skills mirror threat model
• .gitignore — exclude .cursor/ (plugin-generated runtime artifacts)

Test results

403 tests passed
typecheck: clean
build: clean

Security

No API keys, tokens, or personal identifiers in any committed file. The CURSOR_API_KEY is only referenced by name (existing pattern). The skills mirror threat model is documented in SECURITY.md: skill instructions cross the permission boundary once written to disk, deny/ask skills are excluded, and user-owned files are never overwritten.

Limitations

• config.skills.urls (HTTP catalogs) are not supported — documented in README and SECURITY.md.
• cursor_cloud_agent targets a remote repo and does not inherit skills.
• cursor_delegate with a non-session cwd does not mirror skills (but does pass settingSources: ["project"] so pre-existing .cursor/skills/ there still loads).

Verification

Live tested with opencode run --model cursor/composer-2.5 — the Cursor agent successfully discovered, read, and used the mirrored using-agent-skills skill. Standalone verification (node scripts/verify-skill-mirror.mjs) confirms all 46 skills discovered, mirrored, catalogued, and disposed cleanly.