[agent] Hydrate durable Environment status from Parsar retrieve API

[sam2tom]

Context

Parsar Core at immutable commit 0438880ab21aa16d05cb91a4c7f91cc0abc12358
now exposes:

GET /v1/agents/environments/{environment_id}

for the supported self_hosted profile. The response carries durable
pending, connected, disconnected, expired, or failed status plus safe
API-managed installation metadata arrays.

The merged Environment UI in #5 is pinned to Parsar 8cc2898c. It can show
Session Environment identity and Workspace fields, and it can project supported
live SSE states, but after reload/reconnect it cannot recover connection status
durably. It deliberately treats expired as unknown because the old pin did not
expose the public Environment resource.

Observable outcome: selecting, refreshing, or reconnecting a self-hosted Session
shows the latest durable Environment status, including expired, without
inferring executor or provider readiness.

Acceptance criteria

• Add a typed Environment resource projection with exact fields id,
  object, type, status, files, plugins, and skills.
• Keep resource status (pending, connected, disconnected, expired,
  failed) distinct from Session Environment event status (pending,
  ready, connected, disconnected, failed); do not combine them through
  an unchecked string cast.
• Add a client retrieve method for
  GET /agents/environments/{encoded_environment_id} using the existing
  authentication, beta header, error, and abort contracts.
• Retrieve durable Environment state only for a valid self_hosted
  Environment ID after the current Session identity is known; do not query
  none or unknown Environment variants.
• Integrate retrieval with stream recovery: buffer accepted live events,
  apply the durable Session/Items/Environment snapshot, then apply newer
  buffered events.
• Fence late reads and events by Core generation, Session ID, Environment ID,
  request revision, and abort signal.
• Render durable expired as a distinct terminal state after initial load,
  refresh, and reconnect, even when no corresponding SSE event exists.
• Preserve ready as a live-event-only state.
• Clear stale connected/ready claims when Environment retrieval returns
  missing, unsupported, malformed, unauthorized, or network failure; keep
  the conversation usable and show Environment status as unavailable.
• Never treat status, empty files/plugins/skills, HTTP health, or an
  open SSE stream as proof that an executor, native runtime, model, or
  provider is ready.
• Treat empty installation arrays as "no API-managed installations", not as
  host filesystem or capability inventory.
• Add raw fixtures derived from Parsar
  0438880ab21aa16d05cb91a4c7f91cc0abc12358, including every resource status,
  unknown/malformed variants, and exact empty-array projection.
• Update the protocol coverage baseline and immutable documentation links to
  the audited Parsar revision. Record the Environment retrieve delta without
  advertising unrelated newly added Parsar APIs as Web-supported.

Validation

• Client tests for encoded IDs, headers, exact response shape, abort, 400, 401,
  404, 405, 500, malformed payload, and no write/retry behavior.
• Recovery tests for durable expired after reload, live-only ready, a buffered
  newer event winning over an earlier durable read, stale terminal/live state,
  Session switch, Core switch, and Environment identity change.
• UI tests for every durable status, unavailable/error states, accessibility, and
  narrow layout.
• Fixture or local-Core read-only verification of Environment retrieval; no Turn
  or paid model request is required.
• Run pnpm check, git diff --check, and the repository private-state ignore
  check.

Non-goals

• Do not create, list, update, delete, provision, or connect Environments.
• Do not add Files, Plugins, Skills, Artifacts, Vault, or Workspace browsing UI.
• Do not connect the browser to daemon or executor transports.
• Do not modify Parsar Core.
• Do not claim executor/model/provider readiness.
• Do not make a paid model call, deploy, merge, or delete a branch/Worktree.

Dependencies

• Builds on merged PR Add self-hosted Environment and Workspace status UI #15 / closed Issue [agent] Add self-hosted Environment and Workspace status UI #5.
• Links to tracking Issue [tracking] OpenAI Agents API gaps blocked on Parsar Core #8 and narrows its Environment-retrieve row.
• Contract source is Parsar
  0438880ab21aa16d05cb91a4c7f91cc0abc12358; the retrieve implementation entered
  through Parsar commit 5f273776.
• Coordinate the shared application recovery seam and protocol coverage document
  before marking agent-ready.