feat: add opt-in child-session workflow resume

[SSS135]

Summary

Adds explicit opt-in continuation for interrupted workflow child sessions on top of #80's durable replay contract.

• checkpoints the private Pi child session before model prompting
• resumes from the last durable message or tool-result boundary
• stores transcripts in the workflow project's private agent-sessions/ directory
• binds checkpoints to stable execution IDs and call hashes
• starts a fresh continuation path when a saved session is unavailable or incompatible
• reuses compatible paused worktrees
• assigns cleanup ownership for incompatible, removed, terminal, and stopped worktrees
• integrates current upstream edited-script resume behavior

Temporary stack

This draft builds on #80, which builds on #83. Review the child-session commit after those dependencies, or wait until the earlier PRs land and this branch is rebased.

Activation policy

Child-session continuation is enabled through the explicit persistAgentSessions setting at the settings, manager, runtime, and agent boundaries.

Validation

• npm test: 951/951 passed
• focused child-session and durable suites: 283/283 passed
• Biome: passed
• TypeScript build: passed
• git diff --check: passed
• combined shortcut, quality, and spec review completed; call-hash compatibility, shifted-call safety, private-session wording, and active worktree cleanup ownership findings were addressed

Split from and supersedes the child-session and worktree-continuation portion of #74.