feat(gateway): per-channel enabled_toolsets on channel_overrides

[sene1337]

Summary

Family / multi-mode messaging needs different tool surfaces on different chats of the same platform.

Example (BlueBubbles / iMessage):

• Group capture rooms → narrow tools (skills, todo, kanban, no_mcp) so multiparty traffic cannot drive terminal/files/wiki
• Private household desk chat → full tools (terminal, file, web, …) so the agent can actually look things up

Today platform_toolsets.<platform> is all-or-nothing for every chat on that platform. channel_overrides already support per-chat model / provider / system_prompt, but not toolsets.

This PR adds:

platforms:
  bluebubbles:
    channel_overrides:
      "any;+;DESK-GUID":
        enabled_toolsets: [terminal, file, web, skills, todo, kanban]
      "any;+;CAPTURE-GUID":
        enabled_toolsets: [skills, todo, kanban, no_mcp]

Behavior

enabled_toolsets	Effect
absent / empty	platform defaults (platform_toolsets.<platform>)
non-empty list	replaces platform toolsets for that chat only

• Validated through the same _get_platform_tools path as platform config and webhook route toolsets (unknown / platform-restricted names dropped)
• Resolution order: adapter toolsets_for_source (webhooks) → channel override → platform defaults
• Participates in agent cache signature (rebuild on mismatch; no silent mid-turn tool schema swap)

Works for any platform that uses channel_overrides (BlueBubbles, Discord, Telegram, …), not only BlueBubbles.

Related

Builds on the multi-mode messaging story in:

• feat(bluebubbles): optional exact allowed_chat_guids ingress gate #63990 — optional exact allowed_chat_guids ingress gate (which chats may enter the agent at all)

#63990 answers "which conversations are admitted."
This PR answers "once admitted, which tools does this conversation get."

They are complementary; neither replaces the other.

Tests

uv run --with pytest --extra dev pytest \
  tests/gateway/test_channel_override_toolsets.py \
  tests/gateway/test_channel_overrides.py \
  tests/gateway/test_webhook_route_toolsets.py -q
# 26 passed

Docs

• website/docs/user-guide/messaging/index.md — channel override section extended with enabled_toolsets + BlueBubbles example

[sene1337]

Cross-link: builds on multi-mode messaging isolation from #63990 (chat GUID admission). This PR is the per-chat tool surface layer.