feat(agent-interface): add optional provider observation and interactive-terminal contract (#136)

[drewstone]

Problem

The neutral agent-interface environment contract exposed only coarse capability bits (placement/usage/confidential booleans), so a provider had to drop sandbox-SDK methods with no contract slot — identity/placement/resource/billing observation (#136) and terminal/PTY attach (folded-in #138). Braid carries a local adapter to retain data already in the sandbox SDK.

Change (contract phase 1 — additive, optional, no breaking change)

• Observation surface (environment-observation.ts): normalized provider/session identity, lifecycle, credential-free endpoint location, requested+verified placement, requested+effective CPU/RAM/disk/accelerator, current+peak use, per-execution usage+cost, per-env compute billing, account usage/credits/plan/quota/period. Every payload carries a required freshness discriminator (known/stale/unavailable/redacted/unknown) — a value exists only on known/stale, so a missing number is never a measured zero.
• Credential safety: SafeEndpointSchema has no field for a user/password/token/secret and its superRefine rejects a host or scheme that carries embedded credentials.
• Interactive terminal (environment-terminal.ts): a provider-neutral optional capability + typed refs (terminal identity, ordered output frames with a replay cursor, input bytes, resize, attach-vs-logical-resume, detach/close acks, parent-execution identity, fail-closed expiry) — distinct from the interactions (question/permission/plan) channel.
• New optional capability bits on AgentEnvironmentCapabilities; all surfaces optional so a provider declaring none still validates.

Verification

• agent-interface 411/411; pnpm -r check-types green; check:package-artifacts (26 exports / 52 targets); check:control-artifacts 129/129.
• Independent adversarial review confirmed the contract slice is correct; it flagged (as expected) that provider behavior is not implemented — that is the tracked phase 2.

Scope

Contract only. Provider wiring (a CLI-bridge usage/placement readout and a Tangle terminal bridge to the already-live sandbox transport) is phase 2, the same split used for #146 → #154.

Refs #136

[tangletools]

✅ Auto-approved drewstone PR — a9716922

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 · 2026-08-13T01:26:22Z}