review: synthesize [!NOTE] informational tier with #644 alert judiciousness — 4-callout visual ladder + approved Fix-gate (#653)

* review: NOTE-tier callout + `actionable` flag to suppress Fix buttons

Adds an `actionable` parameter to the `create_pull_request_review` tool
(defaults true) so the agent can opt out of the Fix-it/Fix-all/Fix-👍s
footer affordance on informational reviews. Threaded through
`createAndSubmitWithFooter` so the buttons are omitted when
`actionable: false`.

Updates `Review` and `IncrementalReview` mode prompts with a 4th tier:
`> [!NOTE]` + `actionable: false` for mergeable, FYI-style observations
(prior feedback addressed cleanly, minor stale doc reference, etc.).
Calibration note: `[!IMPORTANT]`/`[!CAUTION]` are reserved for findings
that warrant code changes, because that's what trains users to click
Fix. `[!NOTE]` reviews must not carry inline comments — if a point is
concrete enough to anchor to a line, upgrade the whole review tier.

* review: drop redundant `actionable` flag, key Fix buttons off `approved`

`approved` already encodes "this PR is mergeable, nothing for the Fix
button to act on" — `actionable` was a second flag carrying the same
signal. Drop it from the tool schema and `FooterOpts`; the footer gate
stays `if (!opts.approved)` (unchanged from pre-PR behavior, with a new
comment documenting the UX rationale).

NOTE-tier reviews now use `approved: true` + `> [!NOTE]` body instead of
`approved: false` + `actionable: false`. For repos with
`prApproveEnabled: false`, the runtime already downgrades APPROVE to
COMMENT, so the GitHub-side shape is identical to the prior design.

* review: address Pullfrog feedback — drop ambiguous parenthetical + update postRun nudge

- Review-mode calibration: drop the "(or no callout at all)" parenthetical
  that didn't map cleanly to a bullet; replace with explicit "both the
  `[!NOTE]` tier and the 'no actionable issues' tier below use approved:
  true" so the bullet-list anchor is obvious.
- `buildUnsubmittedReviewPrompt` (Review mode): the fallback nudge for
  unsubmitted reviews now defers to the mode prompt's tier matrix and
  acknowledges that `> [!NOTE]` informational reviews submit with
  `approved: true` alongside the canonical "No new issues found." path.
  Previously the nudge only described the pre-NOTE binary world.

mcp/review.ts

@@ -314,7 +314,7 @@ export const CreatePullRequestReview = type({
     .optional(),
   approved: type.boolean
     .describe(
-      "Set to true to submit as an approval. ONLY when the review contains no actionable feedback — neither inline comments nor actionable content in the body. Defaults to false (comment-only review). Rejections are not supported."
+      "Set to true to submit as an approval. Use for both 'no issues found' and informational `> [!NOTE]` reviews where the PR is mergeable as-is and nothing in the body warrants code changes — approving also suppresses the Fix-button footer affordance so users don't dispatch a fix run on non-actionable feedback. Reserve approved: false for `> [!IMPORTANT]` (recommended changes) and `> [!CAUTION]` (critical) reviews. Defaults to false (comment-only review). Rejections are not supported."
     )
     .optional(),
   commit_id: type.string
@@ -842,6 +842,9 @@ async function createAndSubmitWithFooter(
   // API_URL is misconfigured, and future footer-building changes could
   // introduce new throw paths. keep the whole body wrapped.
   try {
+    // Fix buttons are suppressed on approving reviews — those are mergeable
+    // by definition (either "no issues found" or `> [!NOTE]` informational
+    // observations), so dispatching a fix run would be a UX trap.
     const customParts: string[] = [];
     if (!opts.approved) {
       const apiUrl = getApiUrl();