DM-parse error should hint at recovery (airc msg @peer error message)

[joelteply]

Symptom

When `airc msg` is called with a leading `@peer` and an empty/missing message body, the error is:

```
ERROR: Usage: airc send @peer
```

This tells the user the SHAPE of correct DM usage, but doesn't address the most common reason this fires: the user wanted an in-channel @-mention, not a DM (issue #141 root cause). The maintainer just hit this acknowledging the bug-finder.

Fix sketch

Update the error message to point at both intents:

```
ERROR: '@' is interpreted as a DM target with the rest of the args as the message body.

for a DM: airc msg @ ""
for an in-channel mention: airc msg ": " (use a colon, no leading @)
```

One-line code change in `cmd_send`'s usage path. Doesn't fix the underlying #141 design tension (some users will still want IRC-style @-mentions); it just makes the recovery cheap when they hit the parse.

Why this matters

Pointer-to-recovery is the cheapest UX win class — turns a "what did I do wrong?" stall into a "ah, that's the syntax" continuation. Continuum-b741's review (2026-04-27) called this out as low-priority but a class of cheap fixes worth doing.

Out of scope for #144

Filed for next iteration; not blocking main merge.