fix: flow message silently stored a flag as the body

[anshulsao]

The bug

flow message took the body as a blind positional (args[1]) with no flag-guard, and Go's flag.Parse stops at the first positional. So any flag written before the body became the literal body, and the real text (and --urgent) were silently dropped:

flow message tgt --urgent "real body"   → body stored = "--urgent"   (text lost, not urgent)
flow message tgt --body   "real body"   → body stored = "--body"     (text lost)
flow message tgt "real body" --urgent   → body = "real body", urgent=1   ✓ (only this exact order worked)

Agents hit this constantly — they write the flag first (natural CLI order) or invent a --body flag — so real messages were being corrupted and delivered as literal --urgent / --body, with no error.

The fix

Rewrite the arg parser to be order-independent and give the body a real name:

• --urgent recognised anywhere in the args
• a real --body / -m / --message flag for the text (plus --body=…)
• -- to end flag parsing (so a body may start with -)
• unknown flags now error instead of silently becoming the body

All three natural forms now work:

flow message tgt "body" --urgent
flow message tgt --urgent "body"
flow message tgt --body "body"

Tests

• New TestMessageParsingToleratesFlagOrderAndBodyFlag covers flag-first, --body, positional, and the unknown-flag rejection (nothing stored).
• Updated TestMessageRejectsFlagAddressAndClosedTasks: a known flag before the address is now valid (address = first positional); the real protection is that unknown flags error. Closed/done-task rejection unchanged.
• Full suite green.

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