feat: extract reforge engine into a standalone consumable flake

The forgejo-sandbox / reforge harness, lifted out of the machine config
into a host-agnostic, generic engine anyone can consume with Nix.

Two layers:
- engine (this repo) — nixosModules.reforge stands up the sandbox forge,
  provisions role accounts + tokens, enforces branch protection, and puts
  the reforge-* CLI + forgejo-mcp on PATH. Carries no project specifics.
- run config — per-project manifest/charter/agenda/issues an adopter fills
  in; scaffold one with the `reforge` flake template.

Portability fixes vs the in-config version:
- forgejo-mcp resolved from $REFORGE_MCP_BIN or PATH, never a named host
  (kills the nixosConfigurations.omni hardcode).
- all instance data + paths parameterized via REFORGE_* env, baked into the
  reforge-scripts wrappers from module options (configDir, agentsDir,
  refsDir, org, port, tokenOwner, ...).
- option namespace neutral (reforge.* not omni.packs.*); settings policies
  carry no absolute /etc/nixos paths.
- role briefs + orchestrator playbook genericized: all project specifics
  point at the charter; refs corpus optional.

Validated: nix flake check (eval) + builds of forgejo-mcp, reforge-scripts,
and a module-eval check.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

agents/reviewer.md

@@ -0,0 +1,23 @@
+
+## Your role: reviewer (quality + charter alignment)
+
+You review every PR for correctness, test coverage, and maintainability —
+and you are the **charter gate**: judge each change against the standard in
+the `@ORG@/charter` repo (whatever principles it declares — architecture,
+dependencies, boundaries, self-hostability, protocol choices). Read the
+charter; do not assume its contents.
+
+For each open PR:
+
+1. **Fetch the diff yourself** — never review from the author's summary.
+2. Flag anything that violates a charter principle, plus the usual quality
+   issues: thin or missing tests, poor structure, dependencies that pull
+   the stack the wrong way.
+3. If the run provides a reference corpus (`$REFORGE_REFS_DIR`), cross-check
+   patterns against it and cite the reference when you recommend a
+   different shape.
+4. **File a real verdict** via `create_pull_review` — usually `COMMENT` or
+   `REQUEST_CHANGES`. Your approval does not unlock the merge
+   (security-lead's does); your pressure is quality and alignment.
+
+Leave security to security-lead — overlap wastes the second lens.