feat(gateway): unified thread context hook in BasePlatformAdapter

[Tranquil-Flow]

What does this PR do?

Adds fetch_thread_context() and has_active_session_for_event() to BasePlatformAdapter, providing a single extension point for platform adapters to fetch thread/conversation history when the bot first enters an existing thread.

• Slack: refactors existing _fetch_thread_context() into the new base class hook (no behavioral change for Slack users)
• Discord: adds fetch_thread_context() override using channel.history(), giving Discord feature parity with Slack
• Other platforms (Telegram, Matrix, Signal, etc.): unaffected — the base default returns None

Key design decisions

• Session liveness check evaluates reset policy — has_active_session_for_event() calls _should_reset() on the entry, so expired sessions (idle timeout, daily reset) still get thread context seeded into the new session
• Commands are not prefixed — is_command() guard in the base call site prevents /reset, /status, etc. from having context prepended, which would break command parsing
• Slack auto-trigger gate aligned — _has_active_session_for_thread() now delegates to the shared has_active_session_for_event(), so the "respond without @mention" check also respects session expiry

Related Issue

Fixes #6708

Related to #6712 (absorbs Discord thread context approach into unified base class pattern); #1953, #2950, #5816 (already fixed for Slack by #5890; this refactors the Slack implementation into a reusable hook); #6345, #7304 (complementary features — agent-side history tool and restart resilience).

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• gateway/platforms/base.py — has_active_session_for_event(), fetch_thread_context(), call site in _process_message_background()
• gateway/platforms/slack.py — fetch_thread_context() override, _has_active_session_for_thread() delegation, removed old inline call site
• gateway/platforms/discord.py — fetch_thread_context() override using channel.history()
• tests/gateway/test_thread_context_hook.py — 18 new tests (base helper, integration, Discord override)
• tests/gateway/test_slack_approval_buttons.py — updated existing mocks for reset policy check

How to Test

1. Slack (no behavioral change expected):
   • Mention the bot mid-thread — should see [Thread context] in the first response
   • Subsequent messages in the same thread should NOT re-fetch context
   • /reset inside a thread should work as a command, not get context prepended
2. Discord (new feature):
   • Create a thread with some messages, then @mention the bot
   • First response should include awareness of prior thread messages
   • Commands like /new inside threads should work normally
3. Session expiry:
   • Let a session expire (idle timeout), then message the bot in the same thread
   • Should re-fetch thread context for the new session

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS 15 (Darwin 24.6.0)

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Screenshots / Logs

18 new + 128 related tests pass; 14 pre-existing failures unrelated

[DiscountDarcy]

We've merged this into our local for two different Hermes installs -- getting the tests to pass wasn't too difficult and would be worth it to the community so we can have Hermes use the thread as a resource before wandering off to check every existing session for tips!

[teknium1]

Thanks for the unified treatment of thread context and for covering reset and command cases.

Problems

• Current main already implements Discord thread history backfill in plugins/platforms/discord/adapter.py:6480-6537, carrying it through MessageEvent.channel_context (plugins/platforms/discord/adapter.py:6589) and injecting it in gateway/run.py:10371-10375. Porting this PR's Discord override would introduce a second history fetch/context block for the same thread turn.
• The Slack target moved to plugins/platforms/slack/adapter.py in 1a38066054752d601b71fc655a3ed6bf4228e2da, so this needs a current-tree port rather than a clean application.
• The Slack reset-policy concern remains valid: plugins/platforms/slack/adapter.py:4042-4043 treats a mapped entry as active without consulting SessionStore._should_reset() (gateway/session.py:1575-1621). Its context prepend also happens before command classification at plugins/platforms/slack/adapter.py:2886-2905.

Suggested changes

• Preserve the existing Discord channel_context path and re-scope the salvage to the Slack plugin's reset-aware session check and command-safe context injection.

Automated hermes-sweeper review.

[Tranquil-Flow]

Closing — current main already implements the Discord thread history backfill and related thread context hooks. This PR's porting step conflicts with the current adapter code; the remaining gap is small and the new shape is already in place. Thanks for the review.