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Summary

• Adds fava-trails install-jj CLI subcommand that downloads and installs the JJ binary (Linux x86_64/aarch64, macOS x86_64/arm64), eliminating the adoption cliff between pip install fava-trails and a working system
• Updates all "jj not found" error messages in cli.py, jj_backend.py, and conftest.py to reference fava-trails install-jj instead of the non-packaged scripts/install-jj.sh
• Adds 6 new tests; all 187 tests pass

Implementation Details

• Platform check first: Windows prints winget install Jujutsu.Jujutsu and exits
• Version-aware skip: exits 0 if installed version already matches target
• --version flag to pin a specific JJ release
• Safe tarball extraction via extractfile() + manual write (no tf.extract() path traversal risk)
• Download uses urlopen(timeout=30) — no hanging on poor networks
• Filesystem errors wrapped with actionable messages
• Post-install PATH check with shell snippet if ~/.local/bin is not in PATH
• Zero new PyPI dependencies (stdlib only)

Test plan

• test_install_jj_skips_if_version_matches — already-installed short-circuit
• test_install_jj_unsupported_platform — Windows winget guidance
• test_install_jj_unsupported_arch — unsupported Linux arch exits 1
• test_install_jj_downloads_and_installs — full download+extract+install flow
• test_install_jj_custom_version — --version flag changes download URL
• test_install_jj_in_help — subcommand appears in --help output
• Existing test_doctor_missing_jj updated for new error text
• Full suite: 187 tests pass

🤖 Generated with Claude Code

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Architect Review

Risk: Low — ~275 LOC of implementation across 4 source files (cli.py, jj_backend.py, conftest.py, test_cli.py). Remaining 90+ files are codev protocol templates from codev update.

Assessment: Clean implementation that follows existing cli.py patterns.

Security:

• Safe tarball extraction via extractfile() + manual write (no tf.extract() path traversal)
• Download URL hardcoded to official github.com/jj-vcs/jj/releases
• Default urllib SSL verification, timeout=30 on download
• No hash verification on downloaded binary — acceptable for v1, worth adding in a follow-up

Code quality: Good error handling with actionable messages, proper platform detection, version-aware skip. Tests mock correctly with no network calls.

Follow-up suggestion: Add SHA256 checksum verification for downloaded tarballs in a future TICK.

APPROVE — builder may merge.

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Architect review