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>

templates/reforge/agenda.md

@@ -0,0 +1,32 @@
+# Run agenda — <run name>
+
+Everything this particular run must address. Group items and give each a
+short id (`A1`, `B2`, …) so issues and reviews can trace back to it. The
+machine-readable twin of this file is `issues.tsv` (what `reforge-kickoff`
+files as issues) — keep the two in sync.
+
+## A. Production-readiness
+
+- **A1** — Security audit per repo (auth, key custody, secrets, injection).
+- **A2** — Bug hunt on the critical paths.
+- **A3** — Refactor opportunities that are cheap now, expensive after
+  release (strict-from-the-start — no compatibility shims pre-launch).
+- **A4** — Test-coverage review against the charter's test harness.
+- **A5** — Synthesize `GAMEPLAN.md`: what stands between this stack and
+  production-ready, ordered, with owners. (PR into the charter repo.)
+
+## B. <project-specific arc>
+
+- *e.g. an architectural evolution the run should evaluate and stage as
+  tracked divergences in Phase B.*
+
+## C. Coherence
+
+- **C1** — Map every inter-component contract (who calls whom, over what).
+- **C2** — Check each contract against the charter's boundary principles.
+
+## Outputs (deliverables of the run)
+
+- Issues traceable to these agenda items.
+- Reviews on every PR (security + charter, plus any project lens).
+- `GAMEPLAN.md` PR'd into the charter repo.