feat(discord): add thread_require_mention for multi-bot threads

By default, once Hermes participates in a Discord thread (auto-created on
@mention or replied in once) it auto-responds to every subsequent message
in that thread without requiring further @mentions. That's the right default
for one-on-one conversations and isolated channel threads.

But it's a confirmed footgun in multi-bot threads. When a user invokes one
bot per turn — addressing Codex first, then Hermes — every other bot in the
thread also fires on every message, burning credits and spamming the channel.
Author has hit this personally in active multi-bot research-team threads.

Add a new `discord.thread_require_mention` config key (env:
`DISCORD_THREAD_REQUIRE_MENTION`), default `false` to preserve existing
behavior. When `true`, the in-thread mention shortcut is disabled and
threads are gated the same way channels are. Explicit @mentions still pass
through as expected.

Mirrors the existing helper shape (config.extra > env > default) and the
existing yaml→env bridge pattern used by `require_mention`.

Changes:

- gateway/platforms/discord.py: new `_discord_thread_require_mention()`
  helper; in_bot_thread shortcut now AND's with `not _discord_thread_require_mention()`
- gateway/config.py: bridge `discord.thread_require_mention` from config.yaml
  to `DISCORD_THREAD_REQUIRE_MENTION` env var (mirrors the existing
  `require_mention` bridge two lines above)
- hermes_cli/config.py: add `thread_require_mention: False` default to
  DEFAULT_CONFIG['discord']
- tests/gateway/test_discord_free_response.py: 4 new tests covering default
  behaviour (in-thread shortcut still works), enabled behaviour (mention
  required in threads), enabled+mentioned (mention still passes through),
  and yaml-via-config.extra path. Also clears DISCORD_* env vars in the
  `adapter` fixture so process-env state from the contributor's shell
  doesn't leak into per-test behaviour.
- tests/gateway/test_config.py: 2 new tests covering the yaml→env bridge
  (both the apply-from-yaml and env-precedence-over-yaml paths)
- website/docs/user-guide/messaging/discord.md: document the new env var
  + config key with multi-bot rationale; cross-link from `auto_thread`
  section

Tested on Ubuntu 24.04.

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

@@ -277,6 +277,7 @@ Discord behavior is controlled through two files: **`~/.hermes/.env`** for crede
 | `DISCORD_HOME_CHANNEL_NAME` | No | `"Home"` | Display name for the home channel in logs and status output. |
 | `DISCORD_COMMAND_SYNC_POLICY` | No | `"safe"` | Controls native slash-command startup sync. `"safe"` diffs existing global commands and only updates what changed, recreating commands when Discord metadata changes cannot be applied via patch. `"bulk"` preserves the old `tree.sync()` behavior. `"off"` skips startup sync entirely. |
 | `DISCORD_REQUIRE_MENTION` | No | `true` | When `true`, the bot only responds in server channels when `@mentioned`. Set to `false` to respond to all messages in every channel. |
+| `DISCORD_THREAD_REQUIRE_MENTION` | No | `false` | When `true`, the in-thread mention shortcut is disabled — threads are gated the same as channels, requiring `@mention` even after the bot has already participated. Use this when multiple bots share a thread and you want each to fire only on explicit `@mention`. |
 | `DISCORD_FREE_RESPONSE_CHANNELS` | No | — | Comma-separated channel IDs where the bot responds without requiring an `@mention`, even when `DISCORD_REQUIRE_MENTION` is `true`. |
 | `DISCORD_IGNORE_NO_MENTION` | No | `true` | When `true`, the bot stays silent if a message `@mentions` other users but does **not** mention the bot. Prevents the bot from jumping into conversations directed at other people. Only applies in server channels, not DMs. |
 | `DISCORD_AUTO_THREAD` | No | `true` | When `true`, automatically creates a new thread for every `@mention` in a text channel, so each conversation is isolated (similar to Slack behavior). Messages already inside threads or DMs are unaffected. |
@@ -302,6 +303,7 @@ The `discord` section in `~/.hermes/config.yaml` mirrors the env vars above. Con
 # Discord-specific settings
 discord:
   require_mention: true           # Require @mention in server channels
+  thread_require_mention: false   # If true, require @mention in threads too (multi-bot threads)
   free_response_channels: ""      # Comma-separated channel IDs (or YAML list)
   auto_thread: true               # Auto-create threads on @mention
   reactions: true                 # Add emoji reactions during processing
@@ -324,6 +326,20 @@ group_sessions_per_user: true     # Isolate sessions per user in shared channels
 
 When enabled, the bot only responds in server channels when directly `@mentioned`. DMs always get a response regardless of this setting.
 
+#### `discord.thread_require_mention`
+
+**Type:** boolean — **Default:** `false`
+
+By default, once the bot has participated in a thread (auto-created on `@mention` or replied in once), it keeps responding to every subsequent message in that thread without needing to be `@mentioned` again. That's the right default for one-on-one conversations.
+
+In **multi-bot threads** where users address one bot per turn, this default becomes a footgun — every other bot in the thread also fires on every message, burning credits and spamming the channel. Set `thread_require_mention: true` to disable the in-thread shortcut and gate threads the same way channels are gated. Explicit `@mentions` still work as before.
+
+```yaml
+discord:
+  require_mention: true
+  thread_require_mention: true    # multi-bot setup
+```
+
 #### `discord.free_response_channels`
 
 **Type:** string or list — **Default:** `""`
@@ -350,7 +366,7 @@ Free-response channels also **skip auto-threading** — the bot replies inline r
 
 **Type:** boolean — **Default:** `true`
 
-When enabled, every `@mention` in a regular text channel automatically creates a new thread for the conversation. This keeps the main channel clean and gives each conversation its own isolated session history. Once a thread is created, subsequent messages in that thread don't require `@mention` — the bot knows it's already participating.
+When enabled, every `@mention` in a regular text channel automatically creates a new thread for the conversation. This keeps the main channel clean and gives each conversation its own isolated session history. Once a thread is created, subsequent messages in that thread don't require `@mention` — the bot knows it's already participating. Set [`thread_require_mention`](#discordthread_require_mention) to `true` to disable this in-thread shortcut for multi-bot setups.
 
 Messages sent in existing threads or DMs are unaffected by this setting. Channels listed in `discord.free_response_channels` or `discord.no_thread_channels` also bypass auto-threading and get inline replies instead.