feat(mcp): mesh_publish tool — surface publish_to / publish_to_direct (#56)

[hartsock]

What this PR does

Surfaces the bus's fire-and-forget pub/sub primitives (Bus::publish_to /
Bus::publish_to_direct) through amesh mcp — previously reachable only
from the Python bindings. Follow-up to #52's out-of-scope note; closes #56.

• New mesh_publish tool — publish a body to a peer on a topic,
  fire-and-forget (no reply). The peer's bus fans it out to its topic
  subscribers. Same dual-mode peer naming as mesh_request: peer (resolve
  over mDNS → publish_to) OR addr+pubkey (direct dial →
  publish_to_direct, no discovery).
• Refactor: the peer-naming resolution (peer xor addr+pubkey + its
  validation) and topic/body extraction are factored out of tool_request
  into shared peer_route / topic_and_body helpers (with a small
  McpPeerRoute enum), so mesh_request and mesh_publish name peers
  identically and can't drift.

amesh listen is not a bus subscriber (raw Endpoint::accept, bypassing the
inbox), so there is no CLI publish verb — the coherent surface is the tool.

Test plan

Local: just check + just cov-ci (75% floor) green on Windows.

• New mcp_server_direct_addr_publishes_to_subscriber
  (agent-mesh-cli/tests/mcp_stdio.rs) — real amesh mcp subprocess
  publishes to an in-process quiet-bound subscriber Bus by explicit
  addr+pubkey over loopback QUIC (no multicast); the subscriber's
  topic broadcast::Receiver receives the body. Subscribe-before-publish
  makes it deterministic; ran 5× back to back, ~0.3s each.
• New mcp.rs unit tests: mesh_publish rejects neither-peer-nor-addr
  and half-specified direct args (confirms it routes through the shared
  helper). Existing mesh_request validation tests still pass unchanged.
• Updated tools/list assertions (4 tools now) in mcp.rs + mcp_stdio.rs.
• cargo test --workspace, cargo clippy --workspace --all-targets -- -D warnings, cargo fmt --all -- --check — all clean.

Out of scope

• An MCP mesh_subscribe (streaming server→client notifications) — a
  larger, separate feature.
• A CLI amesh publish verb (no CLI subscriber exists to receive it).
• Any change to the resolve/discovery path.

Refs #56, #52.