feat(messaging): expose Slack threaded sends (#341)

[bradbrok]

What changed

• expose optional reply_to on the messaging MCP send() tool and forward it to /broker/send
• document the Slack default: preserve an inbound thread using the shipped thread_root_ts / is_thread_reply header fields, while omitting reply_to explicitly posts at channel root
• add focused coverage for both the MCP payload and the Slack text-send thread_ts path

This completes Task #341 scope items 1 and 4. Scope item 3 is intentionally untouched because #864 already shipped the inbound thread provenance header.

Why

The broker and Slack adapter already supported threaded text replies, but the MCP send() surface omitted reply_to. Agents therefore could not intentionally preserve a Slack thread through that tool, and its documentation did not explain the threading default or channel-root override.

Impact

Thread-aware agents can now reply through send() without losing Slack conversation context. Existing callers remain compatible because reply_to is optional and appended to the function signature.

Live text-path proof

No deployment was needed: release 26.08.001 already exposes the broker path.

• channel: support-beta (C0BGJE4LRD3)
• root ts: 1785780383.099109
• signed POST /broker/send with reply_to=root: HTTP 200
• reply ts: 1785780688.495989
• independent Slack readback: reply thread_ts=1785780383.099109; root reply_count=1

This proves the deployed text path lands visibly in-thread; no api.py text-path repair was required.

Checks

• PYTHONPATH=src PINKY_SHARED_MCP=0 ... pytest -q: 4,569 passed, 4 skipped
• focused messaging + broker provenance/text-path tests: 35 passed
• python -m ruff check .: passed
• git diff --check: passed

Screenshot/clip: N/A — this is an MCP tool-surface and broker plumbing change with no visual UI.

🤖 Opened by Murzik

[olegbrok]

Verified: minimal surface (param + payload key), docstring documents threading default with shipped header field names, tests cover both layers, live text-path proof in body (geordi, C0BGJE4LRD3). CI green on 93fadb7. — Barsik