feat(backend/copilot-bot): post intro message when added to a server (#13220)

### Why / What / How

**Why:** When AutoPilot is added to a Discord server, there's no
in-channel signal explaining how to set it up. A new admin has to
*already know* to run `/setup` — otherwise the bot just sits there
silently. That's a real onboarding gap; a welcome message at join time
gives them the instructions in the channel they're already looking at.

**What:** When the bot is added to a server, it posts a one-shot welcome
message with the `/setup` instructions, how to use the bot (mention or
DM), and the available commands.

**How:**
- New `on_guild_join` event in `DiscordAdapter._register_events` — fires
once per server-add.
- Helpers in a small `intro.py` (sibling to `commands.py`) so the
adapter stays focused: `pick_intro_channel(guild)` picks the server's
system channel if writable, falls back to the first text channel the bot
has `send_messages` in, returns `None` if nothing's writable;
`intro_message()` builds the copy. The handler is a no-op when there's
nowhere to post.
- Posted with `AllowedMentions.none()` (no @ping risk).
- The commands list in the message intentionally **omits `/resume`** —
`/resume` is DM-only and would be rejected in a server, so advertising
it there would mislead.

### Changes 🏗️

- `adapters/discord/intro.py` (new) — `pick_intro_channel` +
`intro_message`.
- `adapters/discord/adapter.py` — register `on_guild_join` event; calls
`intro.*` and gracefully no-ops when no channel is writable.
- `adapters/discord/intro_test.py` (new) — covers system-channel
preference, fallback to first sendable text channel, none-writable case,
no-system-channel case, no-`guild.me` case, and that the message
references `/setup`.

### 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] Full Discord-adapter suite green (56 passed), `black` + `ruff` +
`pyright` clean
- [x] Channel-picking logic covered: prefers `system_channel`, falls
back to first sendable, no-op when nothing writable
- [x] `/resume` is **not** in the in-server commands list (it's DM-only)