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)

website/docs/user-guide/messaging/discord.md

@@ -437,6 +437,33 @@ Behavior:
 - If a message arrives inside a thread or forum post and that thread has no explicit entry, Hermes falls back to the parent channel/forum ID.
 - Prompts are applied ephemerally at runtime, so changing them affects future turns immediately without rewriting past session history.
 
+#### `discord.history_backfill`
+
+**Type:** boolean — **Default:** `false`
+
+When enabled, the bot recovers missed channel messages on each `@mention`. With `require_mention: true`, the bot only processes messages that tag it directly — everything else in the channel is invisible. History backfill scans backwards through recent channel history when triggered, collecting messages between the bot's last response and the current mention, and includes them as context.
+
+This is most useful for **shared sessions** (`group_sessions_per_user: false`) where multiple users contribute to the same conversation and the bot needs to see what happened between turns.
+
+```yaml
+discord:
+  history_backfill: true
+```
+
+> **Note:** Messages that arrive *while* the bot is processing (between a trigger and its response) are not captured. This is an accepted simplification — the user can re-send or tag again.
+
+#### `discord.history_backfill_limit`
+
+**Type:** integer — **Default:** `50`
+
+Maximum number of messages to scan backwards when recovering channel context. In practice the scan usually stops much earlier — at the bot's own last message in the channel, which is the natural boundary between turns. This limit is a safety cap for cold starts and long gaps where no prior bot message exists in recent history.
+
+```yaml
+discord:
+  history_backfill: true
+  history_backfill_limit: 50
+```
+
 #### `group_sessions_per_user`
 
 **Type:** boolean — **Default:** `true`