docs: Update documentation for Agent Skills system

Document the new Skills system introduced in recent commits:

- docs/architecture.md: Add Skills System section with discovery
  priority, embedded skills, script extraction, and key types
- docs/skills.md: New comprehensive guide covering SKILL.md format,
  discovery priority, embedded skills, research skill usage, and
  troubleshooting
- README.md: Update Agent Skills section with correct priority order,
  add embedded skills info, research skill usage, and link to Skills
  Guide in Documentation Map
- AGENTS.md: Add skill creation to Adding Features, skill extraction
  to Dangerous Code Paths, and new Skills System Entry Points section

All documentation links validated - no broken links or orphan files.

Agent: lamport

AGENTS.md

@@ -30,6 +30,7 @@
 - **New tool**: Add definition in `tool_definitions.rs`, implement in `tools/`, add dispatch case
 - **New provider**: Implement `LLMProvider` trait in `g3-providers`
 - **New CLI mode**: Add to CLI args, implement handler in `g3-cli`
+- **New skill**: Create `skills/<name>/SKILL.md`, optionally add to `embedded.rs` for binary inclusion
 - **New config option**: Add to `g3-config` structs
 
 ## Dangerous Code Paths
@@ -43,6 +44,7 @@ These areas have subtle bugs if modified incorrectly:
 | **Tool dispatch** | Missing dispatch cases cause silent failures |
 | **Retry logic** | Aggressive retries hit rate limits harder |
 | **Parser sanitization** | Inline JSON can trigger false tool call detection |
+| **Skill extraction** | Version hash mismatch causes stale scripts; path issues on Windows |
 
 ## Do's and Don'ts
 
@@ -82,3 +84,24 @@ The `analysis/deps/` directory contains static analysis artifacts generated by t
 | `limitations.md` | What could not be observed and what may invalidate conclusions |
 
 These artifacts are useful for understanding coupling, planning refactors, and identifying architectural boundaries.
+
+## Skills System Entry Points
+
+The skills system (`crates/g3-core/src/skills/`) provides extensible agent capabilities:
+
+| File | Purpose |
+|------|--------|
+| `mod.rs` | Public API: `Skill`, `discover_skills`, `generate_skills_prompt` |
+| `parser.rs` | SKILL.md parsing with YAML frontmatter validation |
+| `discovery.rs` | Multi-location discovery with priority ordering |
+| `embedded.rs` | Compile-time skill embedding via `include_str!` |
+| `extraction.rs` | Script extraction to `.g3/bin/` with version tracking |
+| `prompt.rs` | Generates `<available_skills>` XML for system prompt |
+
+**Key invariants for skills**:
+- Skill names must be 1-64 chars, lowercase + hyphens only
+- SKILL.md must have valid YAML frontmatter with `name` and `description`
+- Embedded scripts are extracted lazily (on first use)
+- Version tracking uses content hash, not timestamps
+- Higher priority locations override lower (repo > workspace > global > embedded)
+- Script extraction is Unix-only (chmod 755); Windows support is incomplete