feat: configurable label prefixes; /lifecycle /stage /status with Prow defaults; /help and /good-first-issue

[jeefy]

Description

Tier 2 of the Prow parity work. Finishes the label plugin and makes label prefixes configurable, so a repo like cncf/automation gets /status, /level, /triage, /tag, /sub … with zero code.

feat(labels): configurable label sections with exclusive groups

• Any top-level key in .prowlabels.yaml becomes a /<key> command (plus /remove-<key>) when listed in prow-commands. Sections are either a plain list of values or a mapping:

  level:
    values: [sandbox, incubation, graduation, archived]
    exclusive: true    # applying one removes the other level/* labels

• New getLabelConfig loader normalizes both forms; malformed sections fail with a clear message. getArgumentLabels remains as a thin wrapper (existing error strings preserved).
• Dispatcher order: built-in registry → fixed commands → hand-written handlers → dynamic (^/[a-z][a-z0-9-]*$, not a known handler) → the old "could not execute" error for anything else. /remove-<name> resolves to /<name> only for label commands — /remove-assign does not become /assign (pinned).
• One existing assertion changed: the "unsupported command" test used /not-a-command, which is now a valid dynamic name; it uses /not_a_command, and the dashed case is pinned separately as a missing-yaml-key error.

feat: /lifecycle, /stage and /status with Prow defaults

Registry entries with Prow's fixed value sets — frozen|stale|rotten, alpha|beta|stable, approved-for-milestone|in-progress|in-review — all mutually exclusive as in Prow. A yaml section with the same key overrides the defaults (so cncf/automation's different /status values just work).

feat: /help and /good-first-issue

Prow's help plugin, exactly: /help → help wanted; /good-first-issue → good first issue + help wanted; /remove-help removes both; /remove-good-first-issue removes only good first issue. Label names with spaces can't go through the arg-based engine, so these are fixed commands (no yaml involved — asserted no contents fetch).

feat(labels): match allowlist values case-insensitively

/kind Cleanup → kind/cleanup (allowlist casing wins), incl. exclusive-sibling and /remove- comparisons against labels already on the issue.

Dogfood: .prowlabels.yaml gains a mapping-form triage section; /lifecycle /help /triage added to this repo's prow-commands. Docs: labeling.md documents both forms, exclusive, defaults/overrides, and a cncf/automation-style example; commands.md rows added.

Testing

• npm run all green at every commit — 33 files, 371 tests (was 322); coverage 90.3 / 90.7 br / 97.9 fn
• New bundle-harness scenarios against dist/index.js: /level incubation with mapping-form yaml (DELETE sibling + POST), /help (POST, no yaml GET)
• Hermetic under a simulated Actions env; dist/ byte-identical across repeated packs
• Rebased onto main after feat: Prow parity — case-insensitive commands, self-LGTM guard, author close/reopen/cancel, /close not-planned #136; no conflicts

Follow-ups deliberately not in this PR

Tier 3/4 from the gap analysis: approved label + merge gate, do-not-merge/hold naming option, @org/team expansion, /auto-cc, review-event lgtm.