ci(upstream): keep merge report cleanup clean

[code-yeongyu]

Fixes the upstream-agent merge workflow clean-tree failure from Actions run 27799681367.

What changed

• Added scripts/archive-upstream-agent-report.mjs to copy .github/agent/last-merge-report.md into the QA evidence directory without leaving the automation branch dirty.
• If the report was accidentally committed by the merge agent, the helper removes it with a narrow cleanup commit before the workflow clean-tree gate runs.
• Updated upstream-agent-merge.yml to use the helper instead of cp + rm.

Evidence

• RED: local-ignore/qa-evidence/20260619-actions-27799681367/red-existing-report-rm.txt reproduces D .github/agent/last-merge-report.md and old gate exit 1.
• GREEN targeted: node --test scripts/archive-upstream-agent-report.test.mjs passed.
• GREEN surface: local-ignore/qa-evidence/20260619-actions-27799681367/green-archive-report-gate.txt shows the helper preserves evidence and leaves git status --porcelain empty.
• npm run check passed locally and in the pre-commit hook.
• npm run build passed locally.

Known caveat

• npm test was run; it failed on unrelated live-provider/auth-dependent tests (401 invalid x-api-key, missing Cloudflare env) plus existing coding-agent live/RPC/flaky failures. packages/tui tests passed.

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Summary by cubic

Fixes the upstream-agent merge workflow’s clean-tree gate by archiving and cleanly removing the merge report so the branch stays clean and the gate stops failing.

• Bug Fixes
  • Added scripts/archive-upstream-agent-report.mjs to copy .github/agent/last-merge-report.md to the QA evidence dir ($EVIDENCE_DIR or local-ignore/qa-evidence/upstream-agent) and remove the source.
  • If the report was committed, makes a small cleanup commit; if untracked, deletes it without committing. Leaves git status empty for the clean-tree gate.
  • Updated upstream-agent-merge.yml to call the script instead of cp + rm.
  • Added scripts/archive-upstream-agent-report.test.mjs for coverage.

^{Written for commit 3098091. Summary will update on new commits.}