Detect ripgrep and cmake in dev setup and never install without asking

[ciaranra]

Closes #383. Closes #344.

ripgrep (#383)

just lint runs pre-commit run --all-files, whose dependency-integrity-check hook (always_run: true) executes scripts/dependency-integrity-check.sh. That script hard-requires rg — 36 call sites, including --files, --glob exclusion lists, and {40} interval regexes that the script's own comment explains cannot be done in awk. Nothing in pecos setup or just doctor mentioned ripgrep, so the first sign of trouble was a failed lint run.

ripgrep is now a detected dependency: pecos setup reports and offers it, pecos install ripgrep installs it on demand, and just doctor reports it. The install action is cargo install ripgrep --locked — cargo is already required by every Just recipe, and one command covers macOS, Linux and Windows, so no vendored download, checksum table, or platform matrix was added. The script's missing-rg failure now names the fix instead of only the requirement.

Declining is a supported answer everywhere: setup, doctor, and the script all print pecos install ripgrep plus cargo install ripgrep --locked / brew install ripgrep / apt install ripgrep / winget install BurntSushi.ripgrep and a docs link.

cmake (#344)

The setup prompt for cmake already existed and works. The remaining gap was downstream: pecos rust test, check, and clippy all pass --all-features, which enables mwpf, which needs cmake. A developer who declined the cmake prompt hit a raw highs-sys build error with no route back. All three lanes now preflight cmake and fail fast:

Error: Config("`pecos rust test` runs `cargo test -p pecos-decoders --all-features`,
which enables the `mwpf` feature and requires cmake. Install cmake with
`pecos install cmake`. See https://github.com/.../cmake-setup.md")

Consent fix

prompt::confirm returned default_yes when stdin was not a terminal, and install prompts default to yes — so a non-TTY invocation without --yes silently downloaded and installed. It now declines and explains that --yes accepts prompts non-interactively. PromptMode::AcceptAll and DeclineAll are unchanged, and no CI lane relied on the old behavior (no workflow invokes just build, setup-quiet, just setup, or pecos setup).

just build at a terminal prompts as before. Without a terminal it now declines and the build fails later with the existing LLVM guidance, rather than installing unasked.

Verification

• cargo fmt --all -- --check, cargo clippy --locked --workspace --all-targets -- -D warnings, cargo test --locked -p pecos-build -p pecos-cli (43 + 4 + 3 doctests), cargo build --locked -p pecos-cli — all pass.
• pre-commit run --all-files — clean.
• With shims hiding both cmake and rg: pecos setup --no --skip-llvm --skip-cuda prints both decline blocks with all install routes; pecos setup --quiet < /dev/null auto-declines and installs nothing; pecos rust test fails fast with the message above.
• scripts/dependency-integrity-check.sh exits 1 with the new guidance when rg is hidden, and exits 0 unchanged when it is present.

A shared executable::which_in_path helper replaces two near-duplicate PATH lookups. It takes the caller's filename-suffix policy, so LLVM tool discovery stays .exe-only on Windows and cmake keeps its .exe/.bat/extensionless policy.