fix(backend/copilot-bot): only answer other bots when @mentioned (#13174)

### Why / What / How

**Why:** Two AutoPilot deployments (prod + AutoPilot-Dev) sharing a
Discord server fell into an infinite feedback loop. Both were subscribed
to the same thread, so each one kept answering the other's replies —
forever.

PR #13039 ("allow bot-to-bot") swapped the original `if
message.author.bot: return` skip for a self-ID-only check, on the
assumption that the @mention requirement (channels) and
thread-subscription requirement (threads) would bound bot-to-bot
traffic. They don't: a sibling deployment is a *different user ID*, so
the self-ID check never catches it, and once both bots subscribe to a
thread the subscription gate is satisfied for **each** of them.

**What:** `_should_ignore_message` (Discord adapter) now ignores another
bot's message **unless that bot explicitly @mentions us**. Our own
messages are still always skipped; human messages are unaffected.

**How:** Three guard branches:
1. Author is us → ignore (Discord echoes our own sends back over the
gateway).
2. Author is another bot → ignore *unless* `_is_mentioned()` (the bot
@mentioned us).
3. Otherwise (human) → don't ignore.

This kills the silent thread loop — a plain bot reply with no @mention
is dropped — while keeping the intentional bot-to-bot path open: a bot
that explicitly addresses AutoPilot still gets a response.

> Residual note: two bots that @mention *each other* every turn can
still loop. That is a much narrower failure mode and is the deliberate
"bots can talk" path, so it is intentionally left open. A turn-count
circuit breaker can be added later if needed.

### Changes 🏗️

- `_should_ignore_message` (Discord adapter): skip another bot's message
unless it @mentions us; the existing self-skip is preserved.
- Tests: replaced `test_allows_other_bot_message` with
`test_ignores_unmentioned_bot_message` (regression test for the loop)
and `test_allows_mentioned_bot_message`.

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [x] `test_ignores_unmentioned_bot_message` fails against the pre-fix
code (reproduces the loop) and passes after the fix
- [x] `test_allows_mentioned_bot_message` confirms an explicit @mention
from another bot still gets through
- [x] Full Discord adapter suite green (`poetry run pytest
backend/copilot/bot/adapters/discord/adapter_test.py` — 42 passed),
`black` + `ruff` clean