fix: harden Darwin wake lifecycle and root classification

[ohade]

Summary

Harden AMQ wake lifecycle handling on Darwin and fix default .agent-mail base-root classification when unrelated sibling agents/ directories exist.

Changes

• stabilize exact Darwin wake-target reuse and treat zombie wake processes as stopped
• add identity-safe amq wake retire for an exact managed wake, including PID/start-token/boot-ID and lock revalidation before signaling
• preserve mailbox wake-target metadata while safely retiring the managed process
• classify the default .agent-mail base before the sibling-session heuristic, preventing $HOME/.claude/agents from making same-tree sends look cross-tree
• add regression coverage for target mismatch, identity changes, missing locks, stale locks, zombie processes, and poisoned sibling directories

Testing

• make ci passes locally (only blocked because golangci-lint is not installed on this machine)
• make check-skills fmt-check vet test smoke
• new focused Darwin wake-retire and root-classification tests

Related Issues

None.