`apps/mcp-server/src/server.ts` was ~874 lines of sequential
`server.tool(...)` calls — hard to review in a PR, worse to diff. Split
the 23 tool registrations into eight focused modules under `tools/`,
each exporting `register(server, ctx)`. `buildServer` is now a
~20-line registration list that preserves the original registration
order so existing in-memory MCP-client tests and snapshot fixtures keep
passing.

New files:
  tools/context.ts     — `ToolContext` (store, settings, resolveEmbedder)
  tools/shared.ts      — hivemind-context helpers + types
  tools/heartbeat.ts   — active-session heartbeat wrapper
  tools/search.ts      — search, timeline, get_observations, list_sessions
  tools/hivemind.ts    — hivemind, hivemind_context
  tools/task.ts        — task_list, task_timeline, task_updates_since,
                         task_post, task_claim_file
  tools/handoff.ts     — task_hand_off, task_accept_handoff,
                         task_decline_handoff
  tools/proposal.ts    — task_propose, task_reinforce
  tools/profile.ts     — agent_upsert_profile, agent_get_profile
  tools/wake.ts        — task_wake, task_ack_wake, task_cancel_wake,
                         attention_inbox, task_foraging_report

No tool-behavior change. All 17 mcp-server tests (InMemory MCP client +
task-thread suites) pass unchanged; full workspace suite 206/206 green.