feat(computer): make the policy decision point pluggable

[Harbor404]

What this changes

Refs #350.

Adds an additive decision-point seam to the computer gateway:

• exports PolicyDecider, (PolicyContext) => PolicyDecision | Promise<PolicyDecision>
• adds optional ComputerGatewayOptions.decide
• keeps evaluateActionPolicy(options.policy(), context) as the default when decide is absent
• awaits the selected decider inside the existing async govern path

A host can now replace the evaluator without changing the default rules path or the audit/forwarding order. Decider errors propagate unchanged and the action is not carried out. No external policy engine or new public semantics are introduced.

Where it runs

• New state that outlives a request? None. The decider is a factory-level function; each decision uses the request's resolved PolicyContext.
• What happens on the second replica? Each replica runs the configured decider locally and then follows the same resolve → decide → audit → act path. No in-process decision state is added.
• Anything serialised? No new writes or shared state.
• Anything fanned out to a browser? None.
• New listener, port, or schedule? None.

Boundary and audit

• Every acting call still goes through the gateway: resolve, decide, audit, then act.
• Custom refusals use the same audit row and ActionRefusedError path as built-in refusals.
• Nothing new is trusted from the client; the decider receives the server-resolved context.
• Default behavior is unchanged when decide is not supplied.
• Decider errors propagate before the action reaches the computer; no new wall-clock timeout was added in this minimal increment.

Changelog

No deployment behavior changes unless an embedder opts into decide, so no CHANGELOG.md entry is needed.

Proof

• bun test server/tests/computer-policy-decider.test.ts — 5 pass, 0 fail
• bun test server/tests/computer-policy.test.ts server/tests/computer-gateway.test.ts — 116 pass, 0 fail
• bun run typecheck — app, server, worker exit 0
• bun run lint — 983 files checked, no fixes applied
• bunx biome check server/src/computer/gateway.ts server/src/computer/policy.ts server/tests/computer-policy-decider.test.ts — clean

The new tests cover the built-in evaluator, async custom evaluator/context injection, denial, the default fail-closed path with no rules, and error propagation without carrying out the action.