Mobile: DM messages omit recipient `p` tags, so ACP agents never receive them (desktop parity gap)

**Environment**
- Buzz Desktop 0.5.4 (macOS 27.0), self-hosted relay `wss://askie.communities.buzz.xyz`
- Buzz mobile (Flutter), paired by QR to the same identity (`eea98f63…`) — verified same owner pubkey
- 8 `buzz-acp` agents, default `subscribe=Mentions`, `respond_to=owner-only`, no `last_error`
- Source read at `block/buzz@631b05c` (main, 2026-08-03)

**Expected**
A message sent to an agent's DM from the mobile app reaches the agent, exactly as the same message sent from the
desktop app does. `messageMentionPubkeys.ts` states the contract explicitly: *"A DM addresses every other participant,
so it must carry recipient `p` tags even when the composer text contains no `@mention`. Agent harnesses and human
notification subscriptions both rely on those tags."*

**Actual**
Mobile DM messages carry only `[["h", ]]`. `buzz-acp` subscribes with
`{"kinds":[9,46010,40007],"#h":[channel],"#p":[agent_pubkey],"since":…}` (`crates/buzz-acp/src/relay.rs:3190-3196`),
so the relay never delivers them. The agent is silent with no error anywhere — no log line, no `last_error`, nothing
in the UI. The message renders normally on every client, which makes it look like the agent is hung.

**Root cause**
`desktop/src/features/messages/lib/messageMentionPubkeys.ts:12-29` branches on `channel.channelType === "dm"` and adds
`p` tags for all other participants. The mobile send path
(`mobile/lib/features/channels/send_message_provider.dart:52-70`) has no equivalent branch — it only tags explicit
mentions. `send_message_provider.dart:75` is the sole outbound `kind:9` path in the app.

**Minimal repro**
1. Run a `buzz-acp` agent with defaults (`--subscribe mentions`), DM it from Buzz Desktop → it replies.
2. Send the same text in the same DM from the mobile app → no reply, ever.
3. Inspect both events on the relay: the desktop one has `["p", ]`, the mobile one does not.

**Evidence** (same author, same channel, same kind, same `created_at` — one answered, one ignored)
```
IGNORED dbf339b5… tags [["h","79c9f111-…"]]
ANSWERED fa4c69bb… tags [["h","79c9f111-…"],["p","73aff53a…"]] → reply e1067774… 57s later
```
Reproduced again as a live A/B on 2026-08-04 05:00Z: the `p`-tagged event got the harness's 👀 accept-reaction and a
reply in 8s; the untagged one got neither.

**Suggested fix**
Port `messageMentionPubkeys` to `send_message_provider.dart`: when the channel is a DM, seed `resolvedMentions` with
the other participants' pubkeys before normalization.

**Two smaller bugs found alongside**
1. `SendMessage.call` does `mentionPubkeys ?? _resolveMentions(content, channelId)`, but `compose_bar.dart` always
passes a non-null (often empty) list, so `_resolveMentions` is unreachable from the composer. Typing `@Name` by hand
without tapping the autocomplete silently produces no `p` tag. Desktop resolves typed names.
2. Mobile `_buildReplyTags` (`send_message_provider.dart:146-160`) emits only `e` tags; desktop's `buildReplyTags`
(`desktop/src/features/messages/lib/threading.ts:101-127`) also emits `["p", authorPubkey]` plus mention `p` tags.
So replying to an agent from mobile also fails to wake it.

I searched `block/buzz` issues (`gh-axi search issues "repo:block/buzz mobile DM mention p tag agent"` and a second
phrasing) — **0 results**, no existing report.

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