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)

gateway/config.py

@@ -941,6 +941,14 @@ def load_gateway_config() -> GatewayConfig:
                     if isinstance(ntc, list):
                         ntc = ",".join(str(v) for v in ntc)
                     os.environ["DISCORD_NO_THREAD_CHANNELS"] = str(ntc)
+                # history_backfill: recover missed channel messages for shared sessions
+                # when require_mention is active.  Fetches messages between bot turns
+                # and prepends them to the user message for context.
+                if "history_backfill" in discord_cfg and not os.getenv("DISCORD_HISTORY_BACKFILL"):
+                    os.environ["DISCORD_HISTORY_BACKFILL"] = str(discord_cfg["history_backfill"]).lower()
+                hbl = discord_cfg.get("history_backfill_limit")
+                if hbl is not None and not os.getenv("DISCORD_HISTORY_BACKFILL_LIMIT"):
+                    os.environ["DISCORD_HISTORY_BACKFILL_LIMIT"] = str(hbl)
                 # allow_mentions: granular control over what the bot can ping.
                 # Safe defaults (no @everyone/roles) are applied in the adapter;
                 # these YAML keys only override when set and let users opt back