fix: inbox send loses content when called from subprocess (stdin fallback)

[Alan5168]

Summary

Fixes clawteam inbox send losing message content when called from a subprocess or shell script (e.g., via runtime watch --exec).

Problem

When clawteam inbox send is invoked from a script or subprocess, the content argument may not be passed correctly, resulting in an empty string stored in the event log. This breaks the runtime watch --exec pattern where an executor script needs to respond via inbox send.

Solution

Make the content argument optional. When not provided, read from sys.stdin:

content: Optional[str] = typer.Argument(None, help="Message content", metavar="[CONTENT]")
# ...
if content is None:
    content = sys.stdin.read()

This enables the reliable script pattern:

printf 'result content' | clawteam inbox send team agent

Testing

Verified with:

• Direct stdin piping: printf 'HELLO' | clawteam inbox send team agent ✓
• runtime watch --exec loop with executor script: content correctly written to event log ✓

Related Issue

Fixes #130