Move Claude Code hook registration into code-trace setup

[winjer]

First slice of the installer-slimming direction we discussed: move the fragile, growing logic out of install.sh and into the binary, starting with the highest fragility-per-line piece — the Claude Code Stop-hook registration, which was an embedded python3 program.

What changed

• src/setup.rs — a pure register_stop_hook(Value) -> Value transform (migrate legacy ~/.claude/hooks/code-trace, absolute-path, and with-args hooks to the canonical bare code-trace command; dedup; preserve unrelated settings and unrelated Stop hooks; reset malformed hooks/Stop shapes), plus file IO that treats a missing file as empty and refuses to overwrite invalid JSON. Exposed as code-trace setup --register-hook [--settings-file <path>].
• install.sh — register_claude_code_hook now delegates to the binary, dropping the installer's only python3 dependency. The installer no longer needs an interpreter.
• Tests — hook-registration coverage moves from the python-based tests/install_hook_test.sh (deleted) to Rust: 14 unit tests on the transform + 7 integration tests driving the real built binary (CARGO_BIN_EXE_code-trace), including a new invalid-JSON-refusal case.

Behaviour parity

Identical to the old python logic across all previously-covered cases (verified end to end through the sourced install.sh wrapper), with one cosmetic difference: serde_json sorts JSON keys alphabetically vs python's insertion order. Functionally identical — Claude Code reads it as JSON.

Why

The python3 heredoc was an untested mini-program and a hidden dependency (the only thing in the project needing python). In Rust it's type-checked, unit-tested, and interpreter-free. This also establishes the setup subcommand pattern for migrating the remaining install steps (config write, prompts, plugin copy) incrementally.

cargo test (all suites) and cargo clippy --all-targets are clean.

🤖 Generated with Claude Code

[winjer]

Superseded by #14, which collapses #11–#13 into a single commit against main (and fixes the harness Docker build to copy plugin/ for the new include_str!).