fix: share configured group sessions across users

[Jdelg718]

Summary

• Reduce active agent instruction bloat by moving the full AGENTS reference into docs and keeping AGENTS.md concise
• Add shared_group_chat_ids config normalization and session-key propagation across gateway runner/session store/platform adapters
• Fix shared multi-user detection for configured group threads so prompt/context behavior matches shared session keys

Test Plan

• python -m pytest tests/gateway/test_session.py tests/gateway/test_config.py tests/gateway/test_session_hygiene.py tests/gateway/test_session_reset_notify.py -q -o 'addopts=' — 168 passed after rebasing onto latest main
• git diff --check

Notes

• Net active AGENTS.md reduction vs latest main: 1,073 lines / 49,256 chars (~93.84%)
• Unrelated local backup file intentionally left untracked: tools/send_message_tool.py.bak-20260508-0012

[teknium1]

Thanks for the selective shared-group-session direction. The premise remains valid: current main has only the global group_sessions_per_user setting (gateway/config.py:699-701), while build_session_key() still applies that global policy to every non-thread group (gateway/session.py:949-959).

Problems

• Current adapter key paths have moved to plugin adapters and still pass only the global flags, e.g. plugins/platforms/telegram/adapter.py:7618-7622, plugins/platforms/discord/adapter.py:6611-6615, and gateway/platforms/base.py:4627-4631. A salvage must update all current batching/guard paths or one selected chat can use inconsistent keys.
• /resume and session authorization still classify sharing from global settings only (gateway/slash_commands.py:760-775, :915-928). They need the same per-source resolver as session-key creation so a selected shared group is not treated as per-user.
• User docs currently describe only the global switch (website/docs/user-guide/sessions.md:615-625); the new setting needs documented semantics.

Suggested changes

• Salvage the scoped config against the current plugin adapter layout, centralize the effective policy, and apply it to session keys, prompt attribution, batching, and slash-command authorization.
• Split the unrelated BlueBubbles send-path rewrite from tools/send_message_tool.py into separate review scope.

Automated hermes-sweeper review.

[teknium1]

Closing as superseded by #70197 (merged): deferred as an intentional-design question about the per-user session model — needs a maintainer call before any implementation; not a quality judgment.

Thanks for the work — it's credited in #70197's summary.