auto-label: teach prompt about area::ci + test coverage ≠ documentation

[ramkrishna2910]

Follow-up to #2076.

After running the first backfill of open items via #2076's workflow, two small gaps showed up in the prompt:

1. CI / infrastructure items had no label

Four open items (#2116, #2037, #1960, plus one more on the website side) were about CI workflows, runner cleanup, or test infrastructure. None of area::cli, area::installer, area::api, area::tray fit, so the model returned (none).

This PR:

• Adds area::ci to the Area block of SYSTEM_PROMPT with explicit coverage for: CI / GitHub Actions workflows, self-hosted runner infrastructure, and test infrastructure (fixtures, harness, CI cleanup).
• Adds "area::ci" to KNOWN_LABELS so the parser admits it. The area::ci label already exists on the repo (created out-of-band during the backfill triage).
• Verified the at-most-one-area:: rule still applies: area::ci, area::api → just area::ci passes through; area::ci against an existing area::* is dropped.

2. documentation was over-applied to test-coverage items

The model labeled #2032 and #2040 with documentation even though they were a quality bug and a test-coverage gap respectively. The connection seems to be the model treating "documents the missing coverage" or "missing test docs" loosely.

This PR adds an explicit rule to the prompt:

documentation is for human-readable docs only — READMEs, user guides, installation instructions, doc-comments. Items about test coverage, missing tests, test infrastructure, or test harness changes are enhancement (or bug if the test itself is broken), NEVER documentation.

Test plan

• Local parser verification — 5 cases covering area::ci admission, at-most-one-area:: enforcement, and co-occurrence with engine + type labels.
• After merge, no fresh backfill needed — the schedule cron + event triggers will exercise the new prompt on the next opened issue / commented item. Optionally dispatch a dry-run on an area::ci-shaped existing issue to spot-check.

🤖 Generated with Claude Code

[fl0rianr]

Thanks, looks good code-wise; this is a clean follow-up to #2076.

One small note on the test plan / rollout wording: schedule and issue_comment runs are priority-only, so they will not exercise the updated LLM prompt for already-open items. Existing issues that previously got (none) because area::ci did not exist would need a manual workflow_dispatch in all mode, either targeted or as a fresh backfill. Newly opened issues/PRs will pick this up automatically.

I don't think this blocks the code change, but it might be worth adjusting the PR description/test plan so the rollout expectation is accurate. Your call.