feat(backend/copilot-bot): add /resume to pick and resume a past chat (#13219)

### Why / What / How

> **Stacked on #13199 (`/new`)** — both touch `commands.py` and this
needs `sessions.py`. PR base is `fix/copilot-bot-new-command`; GitHub
auto-retargets to `dev` once #13199 merges.

**Why:** With the bot live, AutoPilot DM users have no way to resume an
existing chat from the platform — they're stuck with whatever session
the DM happens to be pointing at. The web app has the chat history; the
bot should let users hop back into a specific past conversation.

**What:** A new DM-only `/resume` slash command. It lists the user's
most-recent AutoPilot conversations in a Discord dropdown (up to 25,
title + last-active relative time). Picking one points the DM's session
cache at that `session_id` so the next message continues that exact
chat. The command is rejected in servers and threads.

**How:**

- **Backend (RPC, server-side):** new `list_user_chats(platform,
platform_user_id, limit, offset)` on `PlatformLinkingManager`. It
resolves the caller's AutoGPT user via `find_user_link_owner(platform,
platform_user_id)` — the same composite-unique-key lookup
`start_chat_turn` already uses for DMs — and calls
`get_user_sessions(owner)` (which filters strictly `where userId =
owner`). The bot only ever passes its caller's Discord id; the bot
process never holds an AutoGPT `user_id`. A slim response model
(`ChatSessionSummary` = `session_id` + `title` + `updated_at`) keeps
`user_id`/credentials off the wire entirely.
- **Bot facade:** `BotBackend.list_user_chats` calls the RPC and returns
`list[ChatSummary]`.
- **Cache write on pick:** `sessions.set_session()` (mirror of
`clear_session` from `/new`) writes the picked id under the same key the
handler reads (`copilot-bot:session:<platform>:<target_id>`), so the
next message in the DM resumes that chat.
- **DM-only enforcement:** `_handle_resume` rejects when
`interaction.guild is not None`. This is the load-bearing privacy guard:
server-context ownership resolves to the server *owner* (who ran
`/setup`), so listing in a server would leak that user's private chats.
Two independent guards: command rejects in guilds *and* the RPC only
ever resolves via the user/DM path (never a server id).
- **Resume backstop:** even if a pick were spoofed with a foreign
`session_id`, `start_chat_turn` re-validates
`get_chat_session(session_id, owner)` and returns `None` on a user-id
mismatch — a foreign session can never be loaded.
- **3-second ack:** the interaction is `defer()`'d immediately so the
RPC + DB roundtrip stays inside Discord's interaction window; the picker
is sent via `followup.send(view=...)`. (First version hit `404 Unknown
interaction` without the defer.)
- **Picker UI:** first interactive component in the bot —
`discord.ui.View` + `discord.ui.Select`, ephemeral, 180s timeout.
Callback delegates to `_resume_to_session` so the cache-set logic is
unit-testable.

### Changes 🏗️

- `platform_linking/models.py` — `ChatSessionSummary` +
`ListUserChatsResponse`.
- `platform_linking/chat.py` — `list_user_chats` (DM-only owner
resolution + scoped session listing).
- `platform_linking/manager.py` — `@expose list_user_chats` + client
method.
- `copilot/bot/bot_backend.py` — `ChatSummary` dataclass +
`list_user_chats` facade.
- `copilot/bot/sessions.py` — `set_session` helper.
- `copilot/bot/adapters/discord/commands.py` — `/resume` slash command,
ephemeral dropdown picker, `_resume_to_session` helper, relative-time
formatter, listed in `/help`.
- Tests across `chat_test.py`, `sessions_test.py`, `commands_test.py`.

### 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] `/resume` in a DM lists my chats; picking one routes the next
message to **that** chat on the platform (verified live)
- [x] `/resume` in a server is rejected with the DM-only guidance
(verified live)
  - [x] `/new` (the dependency on #13199) still works (verified live)
- [x] Full `copilot/bot` + `platform_linking` suites green, `black` +
`ruff` + `pyright` clean
- [x] DM-only is enforced in two places: the command guard and the RPC
owner-resolution path
- [x] Slim response model — no `user_id`/credentials cross the bot ↔
manager wire