feat(extension): expire managed Agent Task policy

[seonghobae]

Partial implementation of #27, stacked on PR #85.

Buyer/security gap

PR #85 makes Agent Task extension admission an exact managed allow-list but deliberately carries no policy freshness. A previously valid managed-extension set must not remain replayable indefinitely after enterprise policy changes.

TDD / RCA sequence

1. Exact test-only head f4521bacf98f052f275fc87d0f75c108f6e096e9 established the missing freshness boundary before production changed. The tests require one explicit half-open validity window, caller-supplied trusted evaluation time, fail-closed invalid/reversed windows, not-yet-valid and expired decisions, exact-boundary expiry, u64::MAX safety, exact extension identity matching, and preservation of the separate Agent capability boundary.
2. Production carries the deterministic implementation: AgentTaskExtensionPolicy stores valid_from and valid_until; evaluate_agent_task_extension receives the trusted evaluation time; invalid windows, not-yet-valid policy and expired policy have separate typed denials; membership is evaluated only after freshness checks; and the policy window is bounded rather than becoming indefinite ambient authority. The implementation does not read or attest a clock, so all time values remain in one caller-attested time domain.
3. After the live prerequisite advanced, exact old head 891e8b7271d4e1cfcda615053efe54ddf4ce2627 failed exact CI run 32024561674, Rust contracts job 95371180991, at cargo check --locked --workspace --all-targets: the prerequisite had hardened ExtensionAccessRequest::new to require canonical Origin plus trusted evaluation time, while this branch-local negative-capability test still supplied the former four-argument request shape.
4. Current exact head ed6d191802d378303d1dd71a742f86592f206639 non-destructively reconciles the live feat(extension): bound Agent Task managed extension admission #85 parent while preserving this PR's freshness semantics. The repair adapts only that negative-capability request to supply a valid controlled HTTPS origin and trusted time, preserving the assertion that managed-profile admission does not mint Agent capability. No production authority or evidence gate was weakened.

Live dependency / stack state

The independently resolved live prerequisite is exact PR #85 head a3cfb7cc763841fa6953b459bd7c6f4047cd6c6d. Current exact #94 head is ed6d191802d378303d1dd71a742f86592f206639.

Fresh compare reports ahead 12 / behind 0, with merge base exactly the live prerequisite. The unique freshness delta is again limited to CHANGELOG.md, crates/originweave-policy/src/lib.rs, and crates/originweave-policy/tests/agent_task_extension_policy.rs. The reconciliation preserves prerequisite native-messaging/core authority changes without promoting any prerequisite review, check, status, coverage, mergeability, or synthetic-merge evidence to this head.

Keep Draft while #85/#83/#82 remain active.

Current exact-head evidence

For unchanged exact head ed6d191802d378303d1dd71a742f86592f206639 on exact live prerequisite a3cfb7cc763841fa6953b459bd7c6f4047cd6c6d:

• CI run 32077245758: success;
• Rust contracts job 95532953003: repository contracts, canonical formatting, locked workspace/all-target check, full tests, strict Clippy, and rustdoc success;
• Production coverage job 95532953056: exact owned production function/line/region/branch measurement and enforcement success;
• GitHub reports the Draft mergeable;
• formal reviews currently returned: none; and
• inline review threads currently returned: none.

Manifest V3, Security/SAST and central/model workflows not returned for this stacked Draft are absent evidence, not passing evidence. No predecessor-head, prerequisite-head, synthetic merge, queued, pending, skipped, cancelled, stale, status-only, or model-only evidence is promoted as current proof.

Truth boundary

This slice governs deterministic managed-extension policy validity time. It does not authenticate an administrator, read Chrome enterprise policy, attest a clock, install/enable/disable extensions, verify extension signatures/update provenance, mutate profiles, grant Agent capability, authorize native messaging, persist policy, or claim managed enterprise extension support complete.

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