ci: add Gitea Actions gate (make gen/check/lint on push/PR)

- .gitea/workflows/lint.yml: on push + pull_request runs make gen, then
  git diff --exit-code (gen-drift check), then make check, then make lint
- requires a registered act_runner (ubuntu-latest label) to execute
- docs: AGENTS.md Quick facts 'no CI' -> CI bullet (runner required, gates
  still run locally); DEV.md stub-harness note clarifies CI is static-gates
  only; AGENT_TODO Done entry + Next item for runner registration
- verified locally: gen idempotent (0 gen-managed files changed), check OK,
  lint 0 FAIL / 0 WARN

DOC/DEV.md

@@ -199,7 +199,7 @@ make lint                     # convention gate (scripts/lint-conventions.sh)
 
 Example (session-learned): `PATH=/tmp/stubs:$PATH SMB_CONF=/tmp/smb.conf bin/pos-share-smb-server share /tmp/media …`.
 
-Stub harnesses are **throwaway by design**: no `tests/` dir and no CI in this repo — build them outside the project (`/tmp/opencode/<tool>-test/`: `stubs/` + `run-tests.sh` with a `check "desc" "expected" "$actual"` helper and a pass/fail count), run them, then leave them in `/tmp`. Only the *pattern* above is worth keeping in the repo.
+Stub harnesses are **throwaway by design**: no `tests/` dir in this repo — build them outside the project (`/tmp/opencode/<tool>-test/`: `stubs/` + `run-tests.sh` with a `check "desc" "expected" "$actual"` helper and a pass/fail count), run them, then leave them in `/tmp`. Only the *pattern* above is worth keeping in the repo. (CI — `.gitea/workflows/lint.yml` — runs the *static* gates `make gen`+`git diff --exit-code`/`make check`/`make lint` on push/PR; it does not run behaviour suites.)
 
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