Implement project doctor diagnostics and docs

[parthashirolkar]

Summary

• add read-only Git project discovery and project doctor checks for policy, compatibility, gitignore, dirty state, task-store writability, and bwrap readiness
• make doctor default to the current working directory for project detection
• document final doctor/update behavior in README and CHANGELOG

Closes #51
Closes #52
Closes #53
Closes #54
Closes #55
Closes #62

Pre-PR checks

• make lint
• make test
• make build

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