docs: update docs for Mumble integration

Add Mumble sections to USAGE.md, CHEATSHEET.md, API.md, README.md.
Mark Mumble done in ROADMAP.md and TODO.md. Update TASKS.md sprint.

docs/API.md

@@ -357,6 +357,87 @@ Same stable attributes and methods as `Bot`:
 
 ---
 
+## `derp.mumble` -- Mumble Adapter
+
+Alternative bot adapter for Mumble via TCP/TLS protobuf control channel
+(text chat only). All TCP routed through SOCKS5 proxy. Uses a minimal
+built-in protobuf encoder/decoder (no external dependency). Exposes the
+same plugin API as `derp.bot.Bot`.
+
+### `MumbleMessage` dataclass
+
+Duck-typed compatible with IRC `Message`:
+
+| Field | Type | Description |
+|-------|------|-------------|
+| `raw` | `dict` | Decoded protobuf fields |
+| `nick` | `str \| None` | Sender username (from session lookup) |
+| `prefix` | `str \| None` | Sender username (for ACL) |
+| `text` | `str \| None` | Message body (HTML stripped) |
+| `target` | `str \| None` | channel_id as string (or `"dm"`) |
+| `is_channel` | `bool` | `True` for channel msgs, `False` for DMs |
+| `command` | `str` | Always `"PRIVMSG"` (compat shim) |
+| `params` | `list[str]` | `[target, text]` |
+| `tags` | `dict[str, str]` | Empty dict (no IRCv3 tags) |
+
+### `MumbleBot`
+
+Same stable attributes and methods as `Bot`:
+
+| Attribute | Type | Description |
+|-----------|------|-------------|
+| `name` | `str` | Always `"mumble"` |
+| `config` | `dict` | Merged TOML configuration |
+| `nick` | `str` | Bot username (from config) |
+| `prefix` | `str` | Command prefix (from `[mumble]` or `[bot]`) |
+| `state` | `StateStore` | Persistent key-value storage |
+| `registry` | `PluginRegistry` | Shared command and event registry |
+
+**Sending messages** -- same signatures, Mumble protobuf transport:
+
+| Method | Behaviour |
+|--------|-----------|
+| `send(target, text)` | TextMessage to channel (HTML-escaped) |
+| `reply(msg, text)` | `send(msg.target, text)` |
+| `long_reply(msg, lines, *, label="")` | Paste overflow, same logic as IRC |
+| `action(target, text)` | Italic HTML text (`<i>...</i>`) |
+| `shorten_url(url)` | Same FlaskPaste integration |
+
+**IRC no-ops** (debug log, no error):
+
+`join`, `part`, `kick`, `mode`, `set_topic`
+
+**Plugin management** -- delegates to shared registry:
+
+`load_plugins`, `load_plugin`, `reload_plugin`, `unload_plugin`
+
+**Permission tiers** -- same model, exact username string matching:
+
+`_get_tier(msg)`, `_is_admin(msg)`
+
+### Protobuf Codec (internal)
+
+Minimal protobuf wire format helpers -- not part of the stable API:
+
+| Function | Description |
+|----------|-------------|
+| `_encode_varint(value)` | Encode unsigned int as protobuf varint |
+| `_decode_varint(data, offset)` | Decode varint, returns `(value, offset)` |
+| `_encode_field(num, wire_type, value)` | Encode a single protobuf field |
+| `_decode_fields(data)` | Decode payload into `{field_num: [values]}` |
+| `_pack_msg(msg_type, payload)` | Wrap payload in 6-byte Mumble header |
+| `_unpack_header(data)` | Unpack header into `(msg_type, length)` |
+
+### Helper Functions
+
+| Function | Signature | Description |
+|----------|-----------|-------------|
+| `_strip_html` | `(text: str) -> str` | Strip HTML tags and unescape entities |
+| `_escape_html` | `(text: str) -> str` | Escape text for Mumble HTML messages |
+| `_build_mumble_message` | `(fields, users, our_session) -> MumbleMessage \| None` | Build message from decoded TextMessage fields |
+
+---
+
 ## Handler Signatures
 
 All command and event handlers are async functions receiving `bot` and