feat(discord): default history backfill on, expand to per-user + threads

Follow-up to snav's PR #25463 contribution: flip default to on, broaden
scope so backfill fires whenever require_mention gates the bot (not just
shared-session channels).

Why:
- The mention-gate creates a session-transcript gap regardless of whether
  the channel is shared or per-user. In per-user sessions, Alice's session
  is still missing other participants' messages and her own pre-mention
  messages — backfill fills both gaps.
- Threads naturally scope to thread-only history because discord.py's
  channel.history() on a thread returns only that thread's messages.
- DMs still skip — every DM triggers the bot, so the session transcript
  is already complete.

Changes:
- hermes_cli/config.py: discord.history_backfill default → true
- gateway/platforms/discord.py: drop the _is_shared gate, keep _is_dm
  skip and _needed_mention gate; env var DISCORD_HISTORY_BACKFILL
  default → 'true'
- cli-config.yaml.example + website docs: update defaults and prose;
  add the DISCORD_HISTORY_BACKFILL / _LIMIT env var rows that were
  documented in the PR description but missing from the env-var table
- tests/gateway/test_discord_free_response.py:
  - flip test_discord_per_user_channel_does_not_backfill →
    test_discord_per_user_channel_backfills_too (new behavior)
  - add test_discord_dm_does_not_backfill (DM skip is invariant)
  - give FakeThread a no-op history() so existing thread tests don't hit
    a fake discord.Forbidden when backfill now fires on threads too

Tests: 160/160 in target files; 400/400 across all tests/gateway/ -k discord.

cli-config.yaml.example

@@ -689,7 +689,7 @@ platform_toolsets:
 #   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: true           # Recover missed channel messages on mention (default: true)
 #   history_backfill_limit: 50       # Max messages to scan backwards (default: 50)
 
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