refactor(queue): port queue show's dequeue diagnosis onto mergify-events

[jd]

queue show's activity-log fallback was the CLI's first /logs
consumer, and last_leave.rs (744 lines) carried the whole endpoint
contract itself: the explicit 90-day window, the newest-first
ordering assumption, the single-page fetch. That contract now has one
home — the mergify-events crate — so this deletes the module and
rebuilds the diagnosis as its queue_leave explain layer:

• the fetch goes through mergify_events::fetch with
  Window::retained and limit: 1; the wire shape is unchanged
  (same filters, same per_page=1), but the window/pagination/
  ordering traps are the shared client's tests' problem now
• the decode types, the renderer (headline, facts block, the API's own
  reason prose, failing checks with job URLs, next step) and the
  no-activity notice move verbatim to
  mergify_events::queue_leave — the two queue-specific traps they
  guard (leave events vs checks_end abort codes, metadata.merged
  telling a merge from a dequeue) are documented there
• queue show keeps its exact behavior: same human wording (the
  PR #N is not in the merge queue line the live smoke tests pin),
  same --json contract (dequeued tri-state, queue_leave
  verbatim, promoted queue_leave_head_sha), same 403 degradation —
  every test that pinned that behavior moved or stayed, none was
  weakened

Net for mergify-queue: -744 lines, and no /logs knowledge left in the
crate.

Part of MRGFY-8363.

Co-Authored-By: Claude Fable 5 noreply@anthropic.com

[jd]

This pull request is part of a Mergify stack:

#	Pull Request	Link
1	refactor(queue): port queue show's dequeue diagnosis onto mergify-events	#1759	👈
2	feat(events): add mergify events, a timeline over the activity log	#1760

[mergify]

Merge Protections

🔴 2 of 6 protections blocking · waiting on 👀 reviews

	Protection	Waiting on
🔴	👀 Review Requirements	👀 reviews
🔴	🔎 Reviews	👀 reviews
🟢	🤖 Continuous Integration	—
🟢	Enforce conventional commit	—
🟢	📕 PR description	—
🟢	🚦 Auto-queue	—

🔴 👀 Review Requirements

Waiting for

• #approved-reviews-by>=2

This rule is failing.

• any of:
  • #approved-reviews-by>=2
  • author = dependabot[bot]
  • author = mergify-ci-bot
  • author = renovate[bot]

🔴 🔎 Reviews

Waiting for

• #review-requested = 0

This rule is failing.

• #review-requested = 0
• #changes-requested-reviews-by = 0
• #review-threads-unresolved = 0

---

Show 4 satisfied protections

🟢 🤖 Continuous Integration

• all of:
  • check-success=ci-gate

🟢 Enforce conventional commit

Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

• title ~= ^(fix|feat|internal|docs|style|refactor|perf|test|build|ci|chore|revert|ui)(?:\(.+\))?!?:

🟢 📕 PR description

• body ~= (?ms:.{48,})

🟢 🚦 Auto-queue

When all merge protections are satisfied, this pull request will be queued automatically.

[jd]

Revision history

#	Type	Changes	Reason	Date
1	initial	2538e85		2026-08-05 12:18 UTC
2	rebase	2538e85 → c4884b2 (rebase only)		2026-08-05 12:18 UTC
3	rebase	6bacff8 → b2faf78 (rebase only)		2026-08-06 08:58 UTC

[jd]

Rebased onto main — no content change.

The base of this stack, #1758, merged this morning as 8bb025c. Mergify had already auto-restacked this PR onto main (c4884b2 → 6bacff8); this push replaces that with the locally rebased commit so the rest of the stack lines up again — the tip #1760 was left CONFLICTING by the merge and is now clean.

6bacff8 → b2faf78 — compare

Verified standalone at this commit (each commit is its own PR with its own CI): cargo test --workspace and cargo clippy --workspace --all-targets clean.