hive-ag3nt: scrub mcp.rs cookies (#716 batch 8)

Last #716 hotspot — scrub-only batch. All substantive prose is
already documented in docs/turn-loop.md::MCP surface (Sub-agent
tools, Manager tools, Tool whitelist, Tool envelope), so the 10
cookies in mcp.rs come out without prose extraction.

- format_agent_meta rustdoc: drop #432 attribution, add docs pointer
  for the running=false / status_text-stale rationale
- get_agent_meta hive/swarm comment: drop #710 attribution
- AgentServer + ManagerServer add-tool warnings: drop #511 attribution
  twice (kept the lockstep invariant)
- edit_schedule description: drop #474 + #478 cookies in user-facing
  tool text
- cancel_loose_end description: drop #250 cookie in user-facing text
- Manager allow-list block: drop the long #444/#235/#467/#472/#474/
  #478/#509 attribution roll, keep the lockstep invariant
- check_send_allowed (<parent> branch): drop #692, replace with the
  actual semantics ("falls back to operator for root agents")
- prompts/system.md: drop #474 cookie from edit_schedule line
- Cargo.toml [[bin]]: drop #598 pre-/post- split history, add docs
  pointer to docs/turn-loop.md::Harness binary shape

cargo check + 56 tests pass.
Closes #716 (last cookie hotspot).

hive-ag3nt/Cargo.toml

@@ -28,11 +28,10 @@ tracing-subscriber.workspace = true
 tempfile = "3"
 
 [[bin]]
-# Unified harness binary (#598). Replaces the pre-#598 split into
-# `hive-ag3nt` (sub-agent) + `hive-m1nd` (manager). Both code paths
-# live here; the binary picks its role at startup from `HIVE_ROLE`
-# (set by `harness-base.nix` from `hyperhive.role` — `"agent"` or
-# `"manager"`). The privilege boundary is enforced server-side at
-# the socket, so shipping both surfaces in one binary is safe.
+# Unified harness binary: both `agent` and `manager` code paths live
+# here; the binary picks its role at startup from `HIVE_ROLE` (set by
+# `harness-base.nix` from `hyperhive.role`). The privilege boundary is
+# enforced server-side at the socket, so shipping both surfaces in one
+# binary is safe. See `docs/turn-loop.md::Harness binary shape`.
 name = "hive"
 path = "src/bin/hive.rs"