refactor(urgency): extract deadline mechanics

[AKarode]

Summary

• create internal/urgency for deterministic urgent-item mechanics
• own Gmail actionability-label parsing, in-shelf score boosts, recency gating, deadline/negation matching, Unicode-safe deadline-centered snippets, and connector envelope stripping
• move five focused tests and add two direct helper/surface tests
• retain human-sender and occurred-time derivation plus shelf assembly, salience tie-breaking, caps, and brief budgeting in Mora

Closes part of #320.

Boundary and behavior

The 48-hour gate, label weights, ordered phrase vocabulary, word boundaries, immediate negators, snippet lead/default length, From-line removal, Unicode rune indexing, and exact clipping/ellipsis behavior remain unchanged. Mora still uses the shared identity classifier/person resolution to supply the human gate; service/no-sender/stale and full brief integration tests remain above the package.

Evidence

• repository tests: 2,306 → 2,308 (five moved, two added)
• Mora Test funcs: 1,758 → 1,753
• full race suite: 292.45s (prior 291.01s)
• Mora race package: 277.698s
• internal/urgency race coverage: 96.9%

Validation

• CGO_ENABLED=0 go build ./... ✅
• go vet ./... ✅
• golangci-lint run ./... ✅ (0 issues)
• full -race -count=1 suite ✅
• focused urgency/brief/T0/T2 gates ✅
• Windows test compile for internal/urgency and internal/mora ✅
• git diff --check ✅

Tier-1 is not invoked for this deterministic helper extraction; the general local harness remains blocked by the documented macOS signing identity prerequisite.