feat(local): clean uninstall that restores configs to pre-spawn state

[AhmedTMM]

Summary

• Snapshots agent config files spawn writes during spawn <agent> local so they can be reverted on uninstall.
• Adds spawn local-restore [agent] to revert config writes outside an uninstall flow.
• Wires a default-on checkbox into spawn uninstall ("Restore local agent configs to pre-spawn state") whenever snapshots exist.

Addresses Spawn CLI Feedback survey response: "you need a clean uninstall that restores configs to how they were prior."

How it works

• Manifest at ~/.config/spawn/local-backups/manifest.json records {destPath, backupPath, existed, agent, timestamp} for each tracked write.
• local/local.ts uploadFile() snapshots its destination before copying, gated on setBackupAgent() so sandbox/Docker mode is skipped (writes happen inside the container).
• local/main.ts snapshots ~/.bashrc / ~/.zshrc / ~/.profile / ~/.bash_profile plus a per-agent list of paths spawn writes via raw shell (currently ~/.claude.json, ~/.claude/CLAUDE.md) up front so files written outside uploadFile() are still recoverable.
• Restore: pre-existing files are restored byte-for-byte; files spawn created from scratch are removed; manifest entries are dropped on success.

Test plan

• bun test — 2075 pass (7 new tests in local-backup.test.ts covering snapshot, restore, idempotency, agent filtering, corrupt manifest, cleanup)
• biome check src/ — 0 errors
• Manual: run spawn claude local on a host with a pre-existing ~/.claude/settings.json, then spawn uninstall → confirm settings.json is byte-identical to the original
• Manual: run spawn codex local on a host with no ~/.codex/, then spawn local-restore codex → confirm ~/.codex/config.toml is removed

🤖 Generated with Claude Code

[AhmedTMM]

Closing — different direction. The right fix for local-cloud config pollution is to sandbox all local runs (Docker/OrbStack) so spawn never writes to the host in the first place, rather than tracking + reverting writes after the fact. The --beta sandbox path already does this; making it the default obsoletes this PR.