Cross-session hold receipts now surface in the transcript instead of just logging

Claude Code can run multiple sessions that talk to each other, and sometimes a message from one session to another has to be held, dropped, or released (parked) rather than delivered right away. Previously, this only produced a debug log line. Now it's tracked more fully:

• Hold/drop/release notices are written directly into the session's own transcript as a visible system message, marked as informational with a warning level, in addition to still being logged.
• Message state changes (held, dropped, released) are tracked in a lookup table.
• On startup, any messages that were already being held from before get their state seeded correctly.