Follow-up to the #68627 cherry-pick (cluster C15 — Slack platform
capability-note accuracy; earliest report/fix: #6545 by @daikeren):
1. Session/prompt stability: the pinned session-context render
(_pinned_session_context_prompt) is keyed by _ephemeral_change_key,
whose contract requires every rendered input to appear in the key.
The new _slack_tools_loaded() gate reads config + the live MCP
registration map, so its state is now hashed into the key exactly
like the existing Discord gate — a gate flip re-renders ONCE (a
legitimate bust); within a session the note stays byte-stable for
the life of the conversation (A/B: the new parity test fails with
this key change reverted, passes with it).
2. Derived, non-overpromising positive note: rather than hardcoding a
capability list that can drift stale again (the original bug class),
the tools-present note tells the agent to consult the actual loaded
Slack tool schemas for supported operations — the schemas ARE the
source of truth, so the note cannot overclaim ops a given Slack
toolset/MCP server doesn't expose (e.g. a read-only history server).
3. Tests: parity test proving a gate flip changes both render and key;
byte-stability test proving three consecutive turns in one Slack
session return the identical pinned object (sha256-equal); autouse
fixture pins the new gate so key<->render parity is env-independent.
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