fix(mosaic): harden Gitea pr merge fallback (#520)

docs/scratchpads/t_301e4e3b-pr-merge-gitea-empty-uid.md

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+# Scratchpad: t_301e4e3b pr-merge.sh Gitea empty-uid fallback
+
+## Task
+
+Implement a narrow hardening in `packages/mosaic/framework/tools/git/pr-merge.sh` so Gitea merges recover from the known non-interactive `tea pr merge` identity failure: `user does not exist [uid: 0, name: ]`.
+
+## Constraints
+
+- Preserve Mosaic policy gates: squash-only, base branch `main`, queue guard unless explicitly skipped.
+- Preserve the existing authenticated Gitea API fallback when no tea login exists.
+- Do not fallback on arbitrary tea failures.
+- Do not expose tokens or credential-bearing remotes.
+- Scope is limited to the merge wrapper plus focused test/support/scratchpad files.
+
+## External issue
+
+- Gitea issue #520: Harden pr-merge.sh Gitea empty-uid fallback
+
+## Plan
+
+1. Add a focused shell regression harness with mocked `tea` and `curl` proving the known empty uid/name failure must fall back to Gitea API.
+2. Watch the harness fail on current code.
+3. Implement helper functions in `pr-merge.sh` for redacted command display, known failure classification, and authenticated Gitea API merge fallback.
+4. Keep unknown `tea` failures blocking by replaying stderr and exiting non-zero.
+5. Run syntax, shellcheck if available, focused regression, and repo quality gates before push/PR.
+
+## Session log
+
+- 2026-05-22: Read Kanban context, Mosaic global/repo instructions, created isolated branch `fix/t_301e4e3b-pr-merge-gitea-empty-uid`, and opened Gitea issue #520 using the Mosaic issue wrapper/API fallback.
+- 2026-05-22: Added regression harness and watched it fail on current behavior with `user does not exist [uid: 0, name: ]`; implemented narrow fallback and verified known-empty-identity fallback, arbitrary tea failure blocking, and no-tea-login API fallback paths.
+- 2026-05-22: Validation passed for `bash -n`, `shellcheck -x`, focused shell harness, `pnpm typecheck`, `pnpm lint`, `pnpm format:check`, and `pnpm --filter @mosaicstack/mosaic test`. Full `pnpm test` exposed an out-of-scope gateway DB setup failure (`relation "messages" does not exist`) in `apps/gateway/src/__tests__/cross-user-isolation.test.ts`.