feat(discord): channel history backfill for multi-user sessions

Adds optional channel-context backfill for Discord shared-channel sessions
so the agent can see recent messages it missed between its own turns
(typically when require_mention=true filters out most traffic).

Previously the agent only saw the @mention message that triggered it, which
led to disorienting replies in active multi-user channels where the
conversation context was invisible. With backfill enabled, a configurable
number of recent messages are fetched per-turn and prepended to the trigger
message as a context block, kept separate from sender-prefix logic so
attribution remains clean.

This re-opens the work from #13063 (approved by @OutThisLife on 2026-04-20,
closed when I closed the branch to address the simpolism:main head-branch
issue plus an ordering bug I caught later in live use). Filing against the
freshly-rewritten problem statement in #13054 so the design is grounded in
the failure mode rather than the implementation shape.

The implementation follows the **push-mode last-self-anchored** design from
the two options laid out in #13054. See the issue for the trade-off
discussion vs pull-mode (#13120 was an earlier closed PR using that shape).
Treating this as a reference implementation — happy to rewrite as
last-trigger anchoring or as a hybrid with #13120 if maintainers prefer.

Changes:

- gateway/platforms/discord.py:
  - new `_discord_history_backfill()` / `_discord_history_backfill_limit()`
    helpers (config.extra > env > default), mirroring the existing
    `_discord_require_mention()` shape
  - new `_fetch_channel_context()` that scans `channel.history()` backwards
    from the trigger to the bot's last message (or limit), formats as
    `[Recent channel messages] / [name] msg / ...`, respects DISCORD_ALLOW_BOTS,
    skips system messages
  - per-channel `_last_self_message_id` cache to narrow the fetch window
    on hot paths (avoids full history scan when the bot has spoken recently)
  - **IMPORTANT**: passes `oldest_first=False` explicitly to `channel.history()`.
    discord.py 2.x silently flips the default to True when `after=` is supplied,
    which would select the EARLIEST N messages after our last response instead
    of the LATEST N before the trigger. In high-traffic windows this would
    return stale tool traces and drop the actual final answer the user is
    asking about. See regression test below. Caught in live use during a
    Codex tool-trace burst on May 13 2026.
- gateway/config.py: discord_history_backfill + discord_history_backfill_limit
  settings + yaml→env bridge
- gateway/platforms/base.py: channel_context field on MessageEvent
- gateway/run.py: prepend channel_context after sender-prefix so the
  [sender name] tag applies to the trigger message alone, not to the backfill
- hermes_cli/config.py: defaults for new discord.history_backfill and
  discord.history_backfill_limit keys
- cli-config.yaml.example: documented defaults
- tests/gateway/test_discord_free_response.py: 7 new tests covering
  cold-start backfill, self-message stop boundary, other-bot filtering,
  cache hot-path narrowing, stale-cache fallback, shared-channel +
  per-user backfill paths, and the ordering regression test
  (`test_fetch_channel_context_cache_uses_latest_window_when_after_set`)
- tests/gateway/test_config.py: yaml→env bridge tests
- tests/gateway/test_session.py: prefix-order edge cases
- website/docs/user-guide/messaging/discord.md: env vars + config keys +
  usage docs

Tested on Ubuntu 24.04 — empirically validated in my own multi-bot Discord
research server for the past three weeks.

Fixes #13054
Supersedes #13063 (closed)

cli-config.yaml.example

@@ -681,6 +681,16 @@ platform_toolsets:
 #     # allowed_chats: ["-1001234567890"]
 #     extra:
 #       disable_link_previews: false  # Set true to suppress Telegram URL previews in bot messages
+#
+# Discord-specific settings (config.yaml top-level, not under platforms:):
+#
+# discord:
+#   require_mention: true            # Require @mention in server channels (default: true)
+#   auto_thread: true                # Auto-create thread on @mention (default: true)
+#   free_response_channels: ""       # Channel IDs where no mention is needed
+#   reactions: true                  # Show processing reactions (default: true)
+#   history_backfill: false          # Recover missed channel messages on mention (default: false)
+#   history_backfill_limit: 50       # Max messages to scan backwards (default: 50)
 
 # ─────────────────────────────────────────────────────────────────────────────
 # Available toolsets (use these names in platform_toolsets or the toolsets list)