feat(review): read-only conversational /ask for non-write users

[devops-thiago]

Problem Statement

Today all commands except /help require write access (CommentCommandService.HELP_TEXT, ManualReviewAuthorizer). Conversational @mention replies on PRs and review threads use the same authorizer (MaintainerReplyService → ManualReviewAuthorizer.isAuthorized), so read-only collaborators cannot ask questions without triggering spend-heavy review commands.

This is an intentional simplicity tradeoff (prevent drive-by AI spend) documented in PLAN.md → Simplicity tradeoffs, but teams with many read-only reviewers (docs, QA, security readers) want a question-only path that does not post findings, suggestions, check runs, or merge-blocking verdicts.

Proposed Solution

Add a read-only ask mode for conversational mentions (not slash commands):

• Eligible users: any collaborator with read access (or higher) on the repo — confirmed via GitHub collaborator permission API (same pattern as write check in ManualReviewAuthorizer, but accept read/triage/write/admin).
• Trigger: bare @bot … mention on a PR comment or review thread (WebhookController.maybeDispatchConversationalMention / review-thread path) where the body is a question (no command verb like review, describe, etc. — reuse TriggerDetector command detection to reject command-shaped mentions).
• Behavior: single conversational reply (MaintainerReplyService) — no check run, no inline findings, no REQUEST_CHANGES, no label mutations. Optional disclosure footer: "Read-only ask — for a full review, a maintainer with write access can run /review."
• Spend controls: honor #42 caps and /pause; consider a separate lower token budget config for ask-only replies.
• 👀 ack: if #315 lands first, read-only asks should not get 👀 (per feat(webhook): 👀 reaction on slash/mention commands (not conversational asks) #315 scope: commands only) — document clearly.

Config knob, e.g. thrillhousebot.review.read-only-ask-enabled (default: false to preserve current fail-closed behavior).

Related issues

• #315 — 👀 on commands only (explicitly excludes conversational asks)
• #322–#323 — other simplicity-tradeoff knobs (same milestone theme)
• #42 — spend cap
• #50 — model config / budgets

Acceptance criteria

• Read-only collaborator can get a conversational answer when read-only-ask-enabled=true
• Read-only user still cannot run /review, /describe, or other write-gated slash commands
• Ask path never creates or updates check runs, PR reviews, or inline finding comments
• Write users unchanged — existing authorized conversational + command behavior
• Config + README documented; tests for auth gate and command-vs-ask routing

Alternatives considered

• Allow /ask slash command — clearer UX but expands command surface; mention-only keeps parity with "ask the bot a question" flow.
• Open conversational replies to everyone — rejected; still need collaborator check to limit spam.

Priority

Important — simplicity tradeoff evolution; target v0.5.0 with other configurability work (#322, #324).

Code of Conduct

• I have searched for existing issues and this is not a duplicate.