refactor(harness): source harness↔model compatibility from @tangle-network/agent-interface

[vutuanlinh2k2]

What

Deletes agent-app's hand-rolled harness↔model compatibility/snapping fork in
src/harness/index.ts and delegates to @tangle-network/agent-interface — the single source of
truth shared with the sandbox-ui pickers and the cli-bridge backends.

Changes

• isModelCompatibleWithHarness, snapModelToHarness, snapHarnessToModel,
  assertHarnessModelCompatible, modelProvider now delegate to agent-interface (same names
  and signatures, so the call sites in src/sandbox/index.ts and src/web-react/... are untouched).
• Removed the duplicated HARNESS_MODEL_POLICIES, PROVIDER_PREFERRED_HARNESS, and the
  HarnessModelPolicy interface. Confirmed nothing outside this file (in any repo) imports them.
• Kept the parts this module rightly owns: the Harness taxonomy (KNOWN_HARNESSES,
  isHarness, coerceHarness, DEFAULT_HARNESS) and the resolveSessionHarness session-lock
  invariant.
• Added @tangle-network/agent-interface as a required peer dep (+ dev), >=0.15.0.

The one seam: as HarnessType

agent-app's Harness is a superset of agent-interface's HarnessType — it carries forge /
cursor, which agent-interface doesn't list (and omits nanoclaw/gemini/aliases). The
delegations cast harness as HarnessType. This is safe: those agent-app-only runners have no
provider lock in agent-interface, so they resolve as router-backed (any model) — the correct
behavior — and the snap helpers only ever return a vendor-locked harness or opencode, all valid
Harness values. Full harness-taxonomy unification (so the cast can go away) is a deliberate
follow-up, not part of this PR.

Why

The harness↔model rule was hand-copied into three diverging places (agent-interface, sandbox-ui,
and this fork). agent-interface is now the source (tangle-network/agent-sdk#8, published as
agent-interface@0.15.0); this makes agent-app import it so the policy can't drift — and the
nanoclaw correction (router-backed) flows from one place.

Verification (against published agent-interface@0.15.0)

• pnpm typecheck — clean (the casts type-check; nothing references the removed symbols).
• vitest run src/harness/index.test.ts — 12/12 pass (compat, snap, assert throw, session lock).
• vitest run src/sandbox/index.test.ts — the harness-lifecycle cases (deletes and recreates on harness mismatch, driveSandboxTurn) pass. The only 3 failures are pre-existing and
  environment-specific (deferred-file tests that shell out and hit a local .bash_profile error —
  unrelated to this change).

Notes

• No version bump/changelog here — that's the chore(release) flow's job. A release is needed so
  gtm-agent can bump to an agent-app that carries this.
• Part of the selector unification tracked in tangle-network/gtm-agent#395. Siblings:
  agent-sdk#8 (merged + published), sandbox-ui#142 (draft), then gtm-agent dep bumps.

Draft until released and validated downstream.