Route Claude Code through Account Pool with per-machine tokens

[SawyerHood]

Stack layer 3 of 3 (top). Prerequisites: #3035 (provider env contribution hook) and #3036 (Account Pool hub) below it in this stack.

Human comments

What was wrong

The Account Pool hub existed, but Claude Code sessions were not routed to it
automatically: the plugin still exposed one global bearer key, had no
per-thread bypass, and could not replace an unauthenticated machine's local
Claude health with the readiness supplied by an enabled pool account. This is
the BB-90 stack layer on bb/account-pool-hub and
bb/provider-env-contribution.

What changed

• Registered the Account Pool's Claude Code environment contribution with the
  exact server-relative hub route and a secret bearer token minted lazily per
  machine. The resolver contributes nothing when no account is enabled or the
  thread is bypassed.
• Replaced the global hub key with per-machine 0600 token files loaded once
  into a single-process cache, constant-time bearer matching across every
  current and grace token, serialized mint/use/rotation updates, a ten-minute
  rotation grace window, and removal of the obsolete global key file. Last-use
  persistence is throttled to once per machine per minute, expired grace tokens
  are pruned with the next write, and status never returns token values.
• Added bb pool token rotate --machine <id-or-name> and
  bb pool bypass <thread-id> [--off], with matching plugin RPC operations and
  discoverability updates in the CLI skill, generated guide source, and
  configuration docs.
• Tracked the last 24 hours of routed threads and made unsafe disablement
  visible in bb pool status, including proxied hosts that are relying on the
  pool, plus a plugin warning log when a disable leaves those threads on hosts
  without usable local Claude credentials. The dispose-time inspection is
  guarded against failures and bounded to two seconds.
• Added the single experimental SDK member
  bb.providers.experimental_contributeEnvHealth, with fake-host support,
  Plugin Guide documentation, API audit notes, API map inventory, and SDK
  version 0.4.45. The server presents an unauthenticated or expired Claude
  provider as ready/Proxied only when the same live plugin also has an env
  resolver and the Account Pool has an enabled account.
• Reused the lower stack layers' env command payload and hub drain. This PR
  does not change the server-to-daemon wire, so it does not bump
  HOST_DAEMON_PROTOCOL_VERSION.

Not in this PR

• BB's plugin SDK has no system-notice append surface or fitting backend thread
  action. bb.sdk.threads.send would create a real provider turn, so this PR
  does not fake the requested disable-time thread notice or bypass action. The
  safety gap is instead explicit in bb pool status and the disable log.
• Disabling a plugin reloads the Claude bridge, which rebuilds the session but
  discards the previous attachment before it can emit the same cosmetic
  rebuild warning used by an in-place bypass change. In e2e thread
  thr_stxrb938ec, the fresh provider.env-resolved event without pool
  entries and the successful LOCAL AGAIN response demonstrate the actual
  disable behavior. The bypass path in that thread does retain the attachment
  and emits the expected rebuild warning.
• A packaged remote daemon cannot be enrolled into an isolated dev app. The
  multi-machine path is verified by the daemon-side runtime test that expands
  serverPath against its loopback machine-auth URL, plus a live 200 response
  from the dev hub's HEAD /api/hello; this does not claim a live remote
  thread.

How you verified

• Added Account Pool regressions for empty resolver/health output, exact
  per-host contributions and secret masking, distinct machine tokens, bypass
  through CLI and RPC, token rotation and grace expiry, the single-process
  cache and last-use write throttle, secret-free status, proxied and
  unauthenticated routed-host warnings, bounded/failure-safe disable logging,
  and the existing authenticated streaming/drain paths. These behaviors had no
  implementation before this change; all 16 tests pass.
• Added server and fake-plugin-host coverage proving health is ignored without
  an adjacent env resolver and marks only credential-blocked provider health
  ready when a contribution is active.
• pnpm exec turbo run typecheck --filter=@get-bb/plugin-sdk --filter=@bb/server --filter=@bb/plugin-api-map --filter=bb-plugin-account-pool --continue — 9 tasks passed.
• pnpm exec turbo run test --filter=@get-bb/plugin-sdk --filter=@bb/server --filter=@bb/plugin-api-map --filter=bb-plugin-account-pool --continue --force — 11 tasks passed; Account Pool 13 tests, SDK 22 files, API map 11 files, and server 227 files / 2205 tests.
• Review follow-up: pnpm exec turbo run typecheck test --filter=bb-plugin-account-pool --filter=@bb/server --filter=@get-bb/plugin-sdk --continue --force — 13 tasks passed; Account Pool 16 tests, SDK 22 files, and server 227 files / 2205 tests.
• pnpm exec turbo run typecheck test --filter=@bb/agent-runtime --continue --force — typecheck passed and 22 files / 319 tests passed, including loopback serverPath expansion and secret event masking.
• node .github/workflows/check-plugin-sdk-version.mjs — passed with the SDK surface change and version bump.
• git diff --check origin/bb/account-pool-hub...HEAD — passed.
• In the isolated dev app, Claude Code thread thr_stxrb938ec returned
  POOL OK through the hub with an expanded loopback URL and masked token in
  provider.env-resolved; quota utilization moved from unknown to 28%/34%.
  Bypass returned DIRECT OK with no pool env and the rebuild warning. Plugin
  disable returned LOCAL AGAIN using local credentials with no pool env, and
  the plugin was re-enabled. Per-machine token rotation also completed, and a
  live hub hello request returned HTTP 200. BB-90 has the reduced JSON evidence
  and both rendered UI screenshots attached.

Part of #1552

Task: BB-90

AGENT GENERATED

[SawyerHood]

CI for this head ran via manual dispatch after the restack (the pull_request trigger did not fire): https://github.com/get-bb/bb/actions/runs/33832575481 — all jobs passed.

AGENT GENERATED