Implements docs/DEVELOPMENT_PLAN.md §3.13 / §3.13a / §3.13b loaders.

Shipped
-------
- skills/frontmatter.ts (115 lines)
  Zero-dep YAML frontmatter parser supporting the subset SKILL.md / sub-agent /
  output-style files actually use: strings (quoted or bare), numbers, booleans,
  flow-style arrays ["a","b"], block-style arrays, simple block objects.

- skills/loader.ts (110 lines)
  4-layer loader: builtin (compiled-in dir) / user (~/.deepcode/skills/) /
  project (<cwd>/.deepcode/skills/) / plugin (<plugin>/skills/, name prefixed
  with `<plugin>:`). Honors `disabled: true` in frontmatter + settings.json
  `skillOverrides[name].disabled`. `buildSkillsDescriptionBlock()` produces
  the system-prompt fragment listing name+description (body loaded on invoke).

- sub-agents/loader.ts (75 lines)
  Loads .deepcode/agents/*.md. Frontmatter spec: name / description / tools[] /
  model / isolation (subprocess|worktree|none) / maxTurns. Supports CLI
  --agents <dir> override via projectDirOverride option. findSubAgent() helper.

- output-styles/loader.ts (105 lines)
  4 built-in styles (default / explanatory / learning / proactive) from
  §3.13b. User → project layer replacement semantics. applyStyle(base, style)
  appends body to system prompt; respects keep-coding-instructions frontmatter.

Top-level @deepcode/core re-exports all M4 surfaces.

Tests
-----
- frontmatter.test.ts  (10) — quoted strings, booleans, numbers, both array
                              styles, block objects, comments, malformed
- loader.test.ts       ( 9) — user/project sources, plugin qualification,
                              tools/effort/model frontmatter, disabled filter,
                              malformed skip, override gate
- sub-agents/loader.test.ts  ( 6) — all frontmatter fields, layer ordering,
                                     projectDirOverride, findSubAgent
- output-styles/loader.test.ts (9) — 4 builtins, user override displaces
                                     builtin, project displaces user,
                                     applyStyle composition

Total: 240 passed / 4 skipped / 0 failed (was 206).

Deferred
--------
- 15 built-in skill markdown FILES (loader finds whatever's on disk; content
  authoring is out-of-band, not engineering work).
- `Skill` tool for invocation (M5 — will inject skill body into next turn's
  system messages).
- REPL plumbing for skill-frontmatter effort overrides (paired with Skill tool).
- `auto` classifier mode (M3c).

Verified
--------
  pnpm typecheck    → green
  pnpm test         → 240 passed / 4 skipped
  pnpm format:check → conformant

Docs
----
- docs/milestones/M4.md — delivery report + deferred items

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>