fix(rust): on-file spawn error names the program and the $PATH search

[thekevinbot]

Fixes #1172

A bare --on-file program missing from $PATH now fails with a message that names the program (argv[0], not the unsplit template), says $PATH was searched, and, when a file by that name exists in the command's working directory, offers the ./ form. Resolution is unchanged: a bare name is never run from the scanned directory.

Before:

dirsql: skipping `notebook.md`: on-file command failed: failed to spawn `extract.py {path}`: No such file or directory (os error 2)

After:

dirsql: skipping `notebook.md`: on-file command failed: failed to spawn `extract.py`: not found on $PATH. Names without a `/` are resolved against $PATH; use `./extract.py` to run `/tmp/claude-1172-manual/extract.py`

A name without a local match prints only failed to spawn nope-xyz: not found on $PATH. A name containing / keeps the OS error, now naming the program.

Rust API: CommandError::Spawn { command, .. } becomes Spawn { program, .. }, and CommandError::NotOnPath { program, local } is new. Migration fragment covers it. No SDK surface changes, so no PARITY.md change and no parity drift.

Not addressed: the issue's note that the trailing parser produced no rows ... cannot infer a schema line is a cascade when every file fails to spawn. That is a separate behavior change and is left for its own issue.

Manual run: built target/debug/dirsql, then in a temp dir with an executable extract.py and notebook.md, ran dirsql --on-file 'extract.py {path}' "SELECT content FROM './notebook.md'" through the Rust binary, the Python launcher (uv run dirsql) and the TS launcher (node packages/ts/dist/cli/dirsql.js). All three print the "After" line above. ./extract.py {path} returns rows.

Changelog / Migrations

• Changelog fragment added under <root>/<pkg>/changelog.d/ for each changed package (packages/rust/changelog.d/2026-09-24-on-file-not-on-path.md)
• Migration fragment added under <root>/<pkg>/migrations.d/ (packages/rust/migrations.d/2026-09-24-on-file-not-on-path.md: CommandError shape change)

E2E Verification

• Ran e2e suites locally for every affected SDK
• Python SDK e2e: pass
• TypeScript SDK e2e: pass
• Rust core e2e (if applicable): pass
• packages/python/e2e-attestations/<branch>.json receipt written if packages/python changed (just e2e-attest-python)
• packages/ts/e2e-attestations/<branch>.json receipt written if packages/ts changed (just e2e-attest-ts)
• Command(s) run: cargo test -p dirsql --features cli, uv run maturin develop then just e2e-attest-python, just e2e-attest-ts
• Result summary: Rust workspace suite green (incl. new command_runner and path_table_on_file_e2e cases); Python e2e 38 passed; TS e2e 18 passed.