feat(agent)!: align skill install with the Agent Skills standard

[powxenv]

What does this PR do?

Aligns agent install with the Agent Skills open standard. Instead of hand-wiring a few agents with bespoke formats, the CLI now writes one canonical skill per project at .agents/skills/<skill>/SKILL.md (the standard's shared directory), and each agent reads it directly (universal agents) or via a symlink back to it. The bundled registry grows from 8 targets to the full standard set (72 agent ids), so any skills-compatible agent works out of the box.

This broadens adoption (works with 72 agents instead of 8), simplifies maintenance (one uniform code path instead of per-agent wrap()/mode/compact-body machinery), and follows a vendor-neutral standard already adopted by the agents users run.

Related issue

Closes #270

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that changes existing behavior)
• Documentation only
• Build / CI / chore

Checklist

• PR targets the main branch.
• Commits follow Conventional Commits
  (feat(...), fix(...), docs(...), …).
• npm run lint and npm run format:check pass.
• npm run typecheck passes.
• npm test passes and coverage stays at or above the 80% gate.
• New behavior is covered by unit tests (mock-based; no network or
  credentials required).
• No secrets, API keys, internal endpoints, or personal data are included.
• User-facing changes are reflected in README.md / DOCUMENTATION.md where
  relevant.

Notes for reviewers

Breaking changes

This restructures the install model, so existing behavior changes for current users:

• Per-agent bespoke formats removed. Codex no longer receives a managed section in AGENTS.md; Windsurf no longer receives a size-capped compact body. Both now read the canonical .agents/skills/ (codex directly, windsurf via symlink), like every other agent. The wrap() / own-file / managed-section machinery is gone in favor of one standard SKILL.md.
• Install output schema changed. agent install results carry a new mode (canonical | symlink) field, and path now reflects the canonical-or-symlink model rather than each agent's bespoke location.
• Existing installs go stale. Skills written in the old format will surface as stale/modified under agent status until refreshed — re-running agent install (with --force where needed) brings them in line.

What's preserved: the claude / kiro / copilot aliases still resolve, and agent install with no --target still defaults to claude-code — so the commands existing scripts use keep working; only the on-disk layout and output change.

Implementation notes

• Model: canonical .agents/skills/<skill>/SKILL.md is the single source of truth. Universal agents (Codex, Cursor, Cline, Gemini CLI, Copilot, …) read it directly; every other agent gets a relative symlink from its own skills folder back to it — no drift between agents.
• Path safety: both the canonical write and the symlink landing walk each path component with lstat and refuse to traverse a planted symlink (exit 5), so a malicious symlink can't redirect a write outside --dir.
• Tests: the install/status/conflict/force/dry-run/path-safety paths are covered exhaustively by unit tests using an in-memory filesystem (deterministic, cross-platform, no real-symlink fragility); coverage on agent.ts is ~92% lines, with the remainder being defensive error handling and real-fs wrappers exercised by the e2e suite.

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[github-actions]

Thanks for the PR, @powxenv! It links an issue, but that issue isn't assigned to you yet: #270 (unassigned). Per our workflow, claim the issue first by commenting /assign on it (the triage bot assigns you automatically). If it's already assigned to someone else, please coordinate with them or pick another issue — unclaimed-issue PRs are not reviewed. After fixing it, edit the PR description or push a commit to re-run this check. See CONTRIBUTING → Contribution model.