feat(harness): add prime to the canonical harness enum

[drewstone]

What

"prime" (PrimeIntellect prime-agent, the RLM fork of the pi line) joins HarnessType / harnessTypeSchema, with:

• Reasoning set: none…ultracode — the fork's --thinking accepts off|minimal|low|medium|high|xhigh|max (verified in the fork's cli/args.ts at v0.7.0); same expressible set as the current pi entry.
• System-prompt controls: replace + append (--system-prompt / --append-system-prompt verified in the fork).
• Router-backed: no provider lock — its models.json custom-provider path runs any OpenAI-compatible endpoint.
• Deliberately distinct from pi: the fork's wire protocol has diverged (daemon schema rev 8→13 with connect-time rejection of pi-line clients), so the two are not interchangeable at run time — documented in the enum's comment.

Changeset: minor.

Why now

Unblocks first-class backend.type: "prime" registration downstream: agent-dev-container#4921 (sdk-provider-prime, ships with the registration path documented against this enum) and the cli-bridge prime backend (drewstone/cli-bridge#122), which accepts a future harness: 'prime' profile pin by structural comparison so it needs no code change when this lands.

Proof

• agent-interface: 347/347 tests pass, tsc --noEmit clean.
• The pinned system-prompt-owners test and the canonical-set test were widened deliberately (that is their purpose).
• git merge-tree --write-tree origin/main HEAD → clean.

[tangletools]

✅ Auto-approved drewstone PR — 235d86be

This PR was opened by the trusted drewstone account.
The full PR reviewer audit still runs separately and will publish findings if it detects issues.

_{tangletools · auto-approval · reason: drewstone_author · 2026-08-06T08:49:13Z}