feat(mosaic): P5 overlay composer — compose-contract + *.local overlays (#604)

Implements R7 + R8 of the Constitution alpha: launcher-composed operator
overlays so a user's *.local deltas reach the model as one pre-merged blob
(DESIGN §3.2), without re-injecting full base prose.

- composeContract(runtime, mosaicHome=MOSAIC_HOME): testable pure fn extracted
  from the launcher prompt assembly; buildRuntimePrompt delegates to it.
- Overlay logic (deltas by value, base files keep residency):
  - USER.local.md -> appended under the # User Profile block (USER is injected)
  - SOUL.local.md + STANDARDS.local.md -> trailing # Operator Overlays section
    (their bases are load-on-demand; only the small delta is injected, so the
    P3 byte-budget tiering is preserved). Whitespace-only overlays ignored.
  - Absent *.local -> base-only, automatically.
- New command: mosaic compose-contract <harness> -> prints the composed blob
  to stdout (inspection / mosaic doctor / diffing / the composer test).
- defaults/AGENTS.md: bare-launch self-load fallback now nudges to relaunch via
  mosaic <harness> (or mosaic doctor) when *.local overlays exist (R7 known-limit).
- compose-contract.spec.ts (7 tests): per-tier anchors, Tier-3 byte-equality
  (injected L0 == CONSTITUTION.md on disk), overlay present/absent, per-harness.

ASSUMPTION: overlays injected as deltas-by-value under labeled sections; bases
keep existing residency. Rationale in docs/scratchpads/p5-overlay-composer.md.

Verified: 7 composer + 26 launch tests pass; tsc-clean on touched files;
prettier clean. Defers to P6: CONTRIBUTING.md + compliance matrix, line-count
CI ceiling, aiguide reconcile, alpha tag.

Refs #604, #542

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EsgTQzV5YUGk1JtCLP4B83

docs/TASKS.md

@@ -45,3 +45,9 @@ Active workstream is **W1 — Federation v1**. Workers should:
 - Status: PR open, awaiting maintainer merge ratification (fleet-governing change).
 - Cut always-injected contract AGENTS+TOOLS+RUNTIME 8,827→4,122 tok (−53%); all 12 hard gates intact.
 - Validation: deterministic gate-checklist PASS; headless A/B thin 7/9 vs monolith 5/9. Detail: scratchpads/contract-thin-core.md.
+
+## P5 — Overlay composer + cross-harness (#604) — feat/p5-overlay-composer
+
+- Status: in progress. R7 (compose-contract) + R8 (cross-harness) + R9 (composer test).
+- `composeContract({harness, mosaicHome})` pure fn + `.local` overlay deltas-by-value; `mosaic compose-contract <harness>` command; AGENTS bare-launch nudge; composer spec (per-tier anchor + Tier-3 byte-equality). Detail: scratchpads/p5-overlay-composer.md.
+- Defer to P6: CONTRIBUTING.md + compliance matrix; line-count CI ceiling; aiguide; alpha tag.