feat(0.21.0): MCP delegations dispatch into caller's fleet workspace

[tangletools]

Summary

agent-runtime-mcp previously created a fresh sibling sandbox for every delegate_code / delegate_research call. Diffs landed on the worker's filesystem and the caller had to pull them back through the structured tool result.

With this change, when the parent sandbox sets TANGLE_FLEET_ID on the MCP server's env, delegations dispatch onto existing machines in the caller's fleet via fleet.sandbox(machineId).streamPrompt(...). The fleet's shared-workspace policy means worker machines mount the same filesystem as the caller — diffs land in-place, no cross-sandbox copy step.

Sibling-sandbox mode is still the default when TANGLE_FLEET_ID is unset.

Detection mechanism

TANGLE_FLEET_ID env var. When present, the bin calls client.fleets.get(fleetId) to resolve a structural FleetHandle ({ fleetId, ids[], sandbox(machineId) }) and builds a fleet-workspace executor around it. Optional TANGLE_FLEET_EXCLUDE_MACHINES skips the coordinator machine from round-robin.

Fleet mode without TANGLE_API_KEY fails loud rather than silently degrading.

Executor abstraction

interface DelegationExecutor {
  readonly client: LoopSandboxClient   // kernel consumes this
  describe(): string                    // stderr boot tag
}

createSiblingSandboxExecutor({ client })
createFleetWorkspaceExecutor({ fleet, excludeMachineIds, selectMachine? })
detectExecutor({ sandboxClient, env, resolveFleet? })

LoopSandboxClient gains an optional describePlacement(box) so the kernel emits a new loop.iteration.dispatch trace event carrying either { placement: 'sibling', sandboxId } or { placement: 'fleet', fleetId, machineId, sandboxId }. Raw Sandbox SDK clients without the method default to sibling — backward compatible.

createDefaultCoderDelegate accepts executor (new) OR sandboxClient (legacy shorthand, wraps in a sibling executor).

What is NOT in scope

• The kernel (src/loops/) is unchanged structurally — only the new trace event variant + the optional describePlacement hook.
• Sibling-sandbox mode remains the default and is fully wired.
• No npm publish — version bumped to 0.21.0 locally, release happens separately.

Test plan

• pnpm typecheck clean
• pnpm test — 235 passed (215 existing + 20 new across tests/mcp/fleet-detection.test.ts + tests/mcp/fleet-executor.test.ts)
• pnpm build clean
• pnpm exec biome check src tests clean
• Real-credential smoke against a fleet-mounted MCP — deferred; requires a live fleet workspace which isn't available in this session
• tax-orchestrator (or other consumer) sets TANGLE_FLEET_ID in AgentProfile.mcpServers[].env and confirms agent-runtime-mcp: fleet-aware delegation: fleetId=... lands on stderr at boot