feat: implement Deferral signal detector

[mvanhorn]

Summary

Implements the Deferral signal detector that the README's Signal anatomy section reserves but says "no detector produces them yet."

Why this matters

The Signal enum at crates/jilog-review/src/signal.rs:11-22 already declares Deferral(DeferralSignal), the title formatter in tracker.rs:81-83 already handles it, and Signal::session_id() / Signal::kind() already cover the arm. The only thing missing was the detector and its hook in run_review. This PR fills the gap and updates the README so the documented count matches the implementation.

Changes

• New detect_deferrals(&[Message], &str) -> Vec<DeferralSignal> mirrors detect_workarounds: a OnceLock<RegexSet> over assistant text with a parallel label table, first-match-wins per message. Patterns cover come back to, defer/deferring, punt on, leave for later, skipping for now, park for now, next session, circle back.
• detect_workarounds and detect_deferrals are independent. A message like "skipping for now" fires both - the tracker files them under separate titles via the existing signal_title formatter. The "skipping for now" overlap test asserts this is intentional rather than double-counting.
• DigestReport gains pub deferrals: Vec<DeferralSignal>; run_review populates it.
• render_digest and write_digest gain a deferrals: &[DeferralSignal] parameter inserted between workarounds and p0_alerts. The existing four render_digest tests pass &[] for the new arg.
• Digest YAML frontmatter always carries deferrals: N (zero acceptable). The ## Deferrals section is unconditional with _No deferrals detected._ placeholder when empty, matching the existing pattern for Corrections/Errors/Workarounds.
• The commands/review.rs summary println! and the --json output (from the previous PR) both include the deferrals count. The JSON shape stays at schema_version: 1; this is an additive key.
• README "Signal anatomy" table gains a Deferral row between Workaround and P0 Alert. Prose at README.md:128 updates from "Four signal types ... Two more (Pattern, Deferral)" to "Five signal types ... One more (Pattern)".

Depends on #1

This branch is built on top of #1 (--json/--since flags), so the diff GitHub renders here includes both commits: #1's 246-line review.rs/query.rs change plus the 215-line deferral work. The two commits are independent in scope; the deferral work just happens to extend the JSON shape #1 introduced.

Easiest review path: merge #1 first, then GitHub will auto-rebase this PR to just the deferral commit. If you prefer the reverse order, the deferral commit can be cherry-picked to main and #1 rebased on top - the file overlap is small.

Testing

• cargo build --workspace passes
• cargo test --workspace passes (105 tests total, was ~70 pre-PR)
• 8 new deferrals_* tests in detectors.rs cover: basic match, first-match-wins, user-role skipped, tool-blocks excluded, no-match returns empty, label emission, the workaround-vs-deferral overlap on "skipping for now", and the digest render path.

[Joi]

Thanks again for the first PR! Merging this after #1.

One quick design question for the code itself, not the behavior:

The workaround/deferral overlap on "skipping for now" — I'm happy with both signals firing (the precise categorization is genuinely useful), but right now the only place the "this is intentional" reasoning lives is in the test name. Could you add a sentence to the detect_deferrals doc comment (and maybe to the README's Signal anatomy section) in a follow-up PR noting that deferrals and workarounds CAN overlap intentionally? It'd save the next reader from wondering whether the double-fire is a bug. Not blocking the merge.

Heads up: I landed a change to render_digest's signature this morning (commit 1ab3a16 — adds an issue_index: &HashMap<String, IssueRef> parameter for bidirectional digest↔tracker linking). Both signature changes are additive but touch the same function, so I rebased your branch on top of main to resolve. Apologies for the timing collision — that's on us.

Really appreciate the careful work.

— Joi