feat(app): the harness switcher in the chat box (#1549)

[aarontrowbridge]

Closes #1549.

What landed, per layer

Engine route (packages/app-bundle/overlay/packages/opencode/src/server/amicode/harness.ts + registration in the instance httpapi server):

• GET /amicode/harness serves {ok, harness, status, options?} — the current harness plus the extension-published registry view (harness-options.json in the ops dir). The engine never computes or hardcodes the registry; a response without options means "no registry published" and the composer control stays hidden (stock-opencode behavior, unchanged).
• POST /amicode/harness ({harness}) validates the id against that published view (it IS the allowlist), no-ops when already current (the watcher is never poked for nothing), and atomically writes {harness, status:"switching"} — the same loopback guard, sibling ok/false shape, and fixed value-free refusals as the solver-mode flip.

Extension (packages/extension/src/harness_switch.ts, harness.ts, extension.ts):

• harnessMenu — the ONE serialization of the registry both fronts render; decideHarnessSwitch — the ONE gate both consult. Unknown ids, needs-setup, and unentitled targets are blocked with their reason; the harness.telaio entitlement read-side gate lives here (disabled-with-reason), not as a hidden spawn-time failure.
• watchHarnessSwitch — the solver watcher's poll + busy-latch discipline over harness.json; the watcher performs the real switch (gate → amicode.harness setting persist → menu republish → amicode.restartServer) and settles status:"ready". A refused or failed switch settles ready at the still-current harness — never a ready file claiming a switch that didn't happen.
• The amicode.selectHarness palette command now renders from the same menu and re-gates through the same decision at confirm time; boot publishes the options file (best-effort, never blocks boot).

Composer (app + session-ui overlay):

• A fourth select-slot (harnessControl) beside agent/model/variant. Entries carry the honest state; disabled entries show their reason; the disclosure copy — "Sessions are harness-local — switching harnesses switches session history" — rides every selectable entry at the point of switching.
• harnessDegraded: under a non-opencode harness the agent/model/variant slots dim inertly with a "Not accepted by this harness" affordance (Harness Contract v1 accepts none); display: contents otherwise — layout byte-identical.
• The harness switch banner (solver banner's twin) narrates switching → restarting → ready, speaks only for switches the app requested, and carries the same stall/ceiling expiry discipline.

Verification (commands + counts verbatim)

• bunx vitest run test/harness.test.ts (extension) — 32 passed (incl. the new menu/gate/two-fronts pin tests)
• bunx vitest run test/harness_switch.test.ts (extension) — 12 passed
• bunx vitest run test/scores (extension, untouched) — 20 files / 323 passed (expected count matches)
• bun run typecheck (packages/extension) — clean
• bun test test/server/amicode-harness.test.ts (materialized engine tree) — 12 passed, 48 expect() calls
• bun test test/server/amicode-connections.test.ts (solver-mode neighbors, materialized) — 103 passed
• bun test --conditions=solid src/components/harness-switch.test.ts src/components/prompt-input/harness.test.ts (materialized app) — 11 passed
• bash packages/app-bundle/scripts/engine_typecheck_gate.sh — OK (only the Base-pin drift: manifest upstream_base_sha stale vs absorbed overlay #1229-tracked base-drift allowlist errors)
• bash packages/app-bundle/scripts/overlay_typecheck_gate.sh — OK across 7 packages (app ui session-ui core schema sdk/js llm)
• node packages/app-bundle/scripts/drift_gate.mjs — PASS (manifest refreshed for the new/changed overlay files, 859 files)
• bun test --conditions=browser src (materialized session-ui) — 433 pass / 1 fail: the editable-diff-view selection-highlight test fails identically with this slice's changes reverted (pre-existing on this base, unrelated CSS topic)
• bunx vitest run (extension full suite) — 3689 passed / 16 failed across 8 files, all pre-existing: the same 8 files (amicode_service_* auth/bootstrap/proxy/wiring + terminal) fail identically at HEAD without this slice (environment-dependent, need a built dist)
• bun test --conditions=solid --preload ./happydom.ts ./src (materialized app) — 1173 pass / 19 fail + 2 errors, all pre-existing: identical failures with the slice's changed files reverted (i18n parity, sync reducers, editor-dom, offset observers)

Notes

• One registry, two fronts is pinned by test: both flows are decideHarnessSwitch → persist; the same inputs land the same persisted harness from either front, and a blocked request persists nothing from either.
• No schema changes, no new session-identity fields; sessions stay harness-local by the existing restart semantics.

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