ccmux v0.1.17

chat-layer follow-ups from acceptance testing: cancellable watchdogs (msg cancel + --task dedup), honest single-source usage, stdin body, --after+--defer trap warning

• ccmux msg cancel <task> — drop a sender's still-undelivered mail for a task (an armed --after
  watchdog or a queued --defer that hasn't fired). Cancellation is a tombstone in the append-only
  ack-log (by: "cancel"), the same log the daemon and the Stop hook already consult — so a cancel
  suppresses delivery in BOTH channels with no new coordination surface and no ledger rewrite.
  Scoped to the sender, so a session can never cancel another's dispatch; already-delivered mail
  can't be un-sent.
• Watchdog dedup by task: re-arming a conditional (--defer/--after) with the same
  (sender, recipient, task) now REPLACES the sender's prior undelivered one instead of piling up a
  duplicate ping — closing the router's "two identical watchdogs both fired" gap. Immediate mail is
  never replaced (it's delivered at once).
• --after + --defer now prints a note on send: the two multiply to "not before T AND only at a
  turn boundary", so a self-watchdog armed that way won't arrive on time in a long turn — a watchdog
  should use bare --after (delivered between tool calls). Not blocked; the flags are compatible.
• ccmux msg reads the body from stdin (echo "…" | ccmux msg <to>) when no inline text is given
  and stdin isn't a TTY — matching the rest of the toolchain.
• ccmux msg --help now shows the full flag set (--defer/--after/--on-behalf-of/cancel). The
  short help and the arg-error usage were two separate strings that had drifted; they now render from
  one source, so they can't diverge again.
• ccmux inbox help clarifies it's the fallback for held/offline mail, not an archive — a message
  already pushed into a pane doesn't sit there.
• Diagnostic: the daemon now logs when it HOLDS a pending message solely because a human is attached
  to the recipient's pane (delivery resumes on detach) — so "the message never arrived" is traceable
  to that transient cause instead of looking like a broken chat.