fix: embed model_overrides in Codex TOML and OpenCode agent files (#2279)

* docs: sync ARCHITECTURE.md command count to 74

commands/gsd/ has 74 .md files; the two count references in
ARCHITECTURE.md still said 73. Fixes the command-count-sync
regression test.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix: embed model_overrides in Codex TOML and OpenCode agent files (#2256)

Codex and OpenCode use static agent files (TOML / markdown frontmatter)
rather than inline Task(model=...) parameters, so model_overrides set in
~/.gsd/defaults.json was silently ignored — all subagents fell through to
the runtime's default model.

Fix: at install time, read model_overrides from ~/.gsd/defaults.json and
embed the matching model ID into each agent file:
  - Codex: model = "..." field in the agent TOML (generateCodexAgentToml)
  - OpenCode: model: ... field in agent frontmatter (convertClaudeToOpencodeFrontmatter)

Also adds readGsdGlobalModelOverrides() helper and passes the result
through installCodexConfig() and the OpenCode agent install loop.

Closes #2256

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat(commands): add gsd:inbox command for GitHub issue/PR triage

inbox.md was created but not committed, causing the command count
to read 73 in git while ARCHITECTURE.md correctly stated 74.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -113,7 +113,7 @@ User-facing entry points. Each file contains YAML frontmatter (name, description
 - **Copilot:** Slash commands (`/gsd-command-name`)
 - **Antigravity:** Skills
 
-**Total commands:** 73
+**Total commands:** 74
 
 ### Workflows (`get-shit-done/workflows/*.md`)
 
@@ -409,7 +409,7 @@ UI-SPEC.md (per phase) ───────────────────
 
 ```
 ~/.claude/                          # Claude Code (global install)
-├── commands/gsd/*.md               # 73 slash commands
+├── commands/gsd/*.md               # 74 slash commands
 ├── get-shit-done/
 │   ├── bin/gsd-tools.cjs           # CLI utility
 │   ├── bin/lib/*.cjs               # 19 domain modules