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/scratchpads/p5-overlay-composer.md

@@ -0,0 +1,43 @@
+# P5 — Overlay composer + cross-harness (compose-contract)
+
+- **Issue:** #604 · **Branch:** `feat/p5-overlay-composer` · **Lineage:** #542 → constitution alpha
+- **Requirements:** R7 (compose-contract) + R8 (cross-harness) + R9 (composer test)
+- **Design of record:** `docs/design/framework-constitution/{DESIGN.md §3.2, PRD.md §4}` (on `feat/framework-constitution-alpha`)
+
+## Locked design (sequential-thinking)
+
+Current `launch.ts` assembly (`buildComposedPrompt`) injects by value: mission + PRD + hard-gate +
+CONSTITUTION + AGENTS + USER + TOOLS + runtime. It does **not** inject SOUL or STANDARDS (those are
+read-on-demand per the gutted AGENTS dispatcher), and has no `.local` overlay support.
+
+**Decision (ASSUMPTION — recorded for the PR):** overlays are injected as **deltas by value** under
+labeled sections; base files keep their existing residency.
+
+- `USER.local.md` → appended directly under the `# User Profile` block (USER is injected).
+- `SOUL.local.md` + `STANDARDS.local.md` → a trailing `# Operator Overlays` section (their bases are
+  load-on-demand, so only the small delta is injected — not the full base prose).
+- **Why:** honors DESIGN §3.2 ("model gets one pre-merged blob, no read-merge ritual") while preserving
+  the P3 byte-budget tiering (don't re-inject large SOUL/STANDARDS prose). Precedence order kept: base
+  layers first, operator overlays at recency.
+- Base-only is automatic when a `.local` file is absent (`readOptional`).
+
+## Plan
+
+| #   | Task                                                                                                   | File                                    |
+| --- | ------------------------------------------------------------------------------------------------------ | --------------------------------------- |
+| 1   | Extract `composeContract({harness, mosaicHome})` pure fn; `buildComposedPrompt` delegates              | `src/commands/launch.ts`                |
+| 2   | Overlay logic (USER.local under profile; SOUL/STANDARDS.local in `# Operator Overlays`)                | `src/commands/launch.ts`                |
+| 3   | `mosaic compose-contract <harness>` command → prints blob to stdout                                    | `src/commands/launch.ts`                |
+| 4   | Bare-launch overlay nudge in self-load fallback                                                        | `framework/defaults/AGENTS.md`          |
+| 5   | `compose-contract.spec.ts`: per-tier anchor, Tier-3 byte-equality, overlay present/absent, per-harness | `src/commands/compose-contract.spec.ts` |
+
+## Deferred to P6
+
+CONTRIBUTING.md + harness×gate compliance matrix; resident line-count CI ceiling; `aiguide` reconcile;
+alpha tag `mosaic-vX.Y.Z-alpha`.
+
+## Status
+
+- [x] Phase scaffold (branch, issue #604, scratchpad, TASKS)
+- [ ] Implementation (tasks 1–5)
+- [ ] prettier + vitest green; PR via wrapper → Lead (rides 0.0.39; 0.0.38 mid-cut)