feat(pr): add Gitea/Forgejo forge support

[chewbakartik]

Intent

Correct the stale forge-merge-watch.md disclosure and PR #1399 Risk Assessment: the autonomous merge path has since been smoke-tested live end-to-end against a real Gitea/Forgejo pull request and passed on the first attempt with no code changes required.

What Changed

• Add Gitea/Forgejo detection and API handling across bin/fm-pr-check.sh, bin/fm-pr-lib.sh, bin/fm-pr-merge.sh, and bin/fm-pr-poll.sh, extending the existing GitHub/GitLab forge support to a third fully-supported forge.
• Replace docs/gitlab-merge-watch.md with a consolidated docs/forge-merge-watch.md covering all supported forges, and update AGENTS.md, README.md, docs/architecture.md, docs/scripts.md, and docs/documentation-audiences.json references accordingly.
• Extend tests/fm-pr-check-security.test.sh and tests/fm-pr-merge.test.sh with Gitea/Forgejo coverage, and correct the forge-merge-watch.md disclosure to reflect that the autonomous merge path has since been smoke-tested live end-to-end against a real Gitea/Forgejo pull request.

Risk Assessment

✅ Low: The change is a docs-only correction (4 insertions/3 deletions in docs/forge-merge-watch.md) that replaces the stale "never exercised" disclosure with an accurate account of a live smoke test; no code changed, no other doc references the old claim, and the corresponding PR #1399 body's Risk Assessment already reflects the same correction.

Testing

This is a pure prose/documentation correction (docs/forge-merge-watch.md, +4/-3 lines, no code touched), so there is no unit-testable behavior change; verification consisted of confirming the stale disclosure text was fully removed (grep found zero remaining occurrences of the old "not exercised"/"smoke-test the actual arm-and-merge path" phrasing), running the doc's own cross-referenced hermetic suite bash tests/fm-pr-merge.test.sh (16/16 pass, matching the doc's claim that the tea merge call is "additionally covered by the hermetic fake-tea tests"), and cross-checking PR #1399 on GitHub (read-only, via gh-axi pr view 1399 --full) to confirm its Risk Assessment section already reflects the same corrected live-smoke-test narrative, satisfying the "and PR #1399 Risk Assessment" half of the intent. The underlying live end-to-end smoke test against a real Gitea/Forgejo instance was performed by the author outside this sandbox (no live forge instance or tea auth available here) and is not independently re-verifiable from this environment; that is expected since the task is to correct the record of an already-performed manual test, not to redo it. Working tree is clean after testing.

Pipeline

Updates from git push no-mistakes

✅ **intent** - passed

✅ No issues found.

✅ **Rebase** - passed

✅ No issues found.

✅ **Review** - passed

✅ No issues found.

✅ **Test** - passed

✅ No issues found.

• grep -n "not exercised\|No live end-to-end\|was not exercised\|smoke-test the actual" docs/forge-merge-watch.md (confirms zero stale/contradictory occurrences remain)
• bash tests/fm-pr-merge.test.sh (16/16 pass — the hermetic suite the corrected doc text cites for tea-merge coverage)
• gh-axi pr view 1399 --full (read-only check that PR #1399's Risk Assessment section matches the corrected disclosure)
• git status --porcelain (clean working tree after testing, no stray artifacts)

✅ **Document** - passed

✅ No issues found.

✅ **Lint** - passed

✅ No issues found.

✅ **Push** - passed

✅ No issues found.

[chewbakartik]

I did an end to end test of this functionality, so I find it odd that the risk assessment flags that as a consideration. I am not actively at my system at the moment, but will instruct that to be updated and reflected in the PR.