feat(hook): add TaskCompleted drain + multi-event install-hook

Wire a fifth pull hook so peer messages also surface between todo
items, not only at user prompts and subagent stops. While here,
extend the manual `install-hook` CLI so it patches the full plugin
hook set (SessionStart/UserPromptSubmit/SubagentStop/TaskCompleted/
SessionEnd) instead of only UserPromptSubmit, mirroring what the
plugin's hooks.json registers. Mailbox name is auto-derived from
stdin, so --name is no longer required.

Also corrects stale docs that claimed SessionStart auto-bootstraps
the watcher — push delivery has been opt-in since the
mailbox-collaborate skill landed.

README.md

@@ -191,7 +191,7 @@ claude-mailbox watch        --block --name <mailbox> [--timeout 25]
 claude-mailbox list
 claude-mailbox status
 claude-mailbox session-announce         # hook helper, reads stdin JSON
-claude-mailbox install-hook   --name <mailbox> [--user|--project]
+claude-mailbox install-hook                    [--user|--project] [--url <url>]
 claude-mailbox uninstall-hook                  [--user|--project]
 ```