Track 3 Contract

This file pins the implementation contract for command and doctor discovery on branch feat/commands-doctor.

Agreed Direction

Commands are exposed through import bindings: gc <binding> <command...>.
Doctor checks contribute to plain gc doctor.
Commands are closed by default with the rest of the import model.
This track keeps command exposure city-scoped; rig imports may contribute doctor checks, but not gc <binding> ... command namespaces.
This track does not invent new extension roots, city-root command exposure, or transitive command re-export behavior.

Command discovery is convention-based under commands/.
Nested directories imply nested command words.
A command leaf is a directory containing run.sh.
Command leaves are terminal: once a directory is treated as a command leaf, discovery does not recurse into its child directories. Child directories under a leaf are treated as local assets, not nested commands.
help.md is optional help content for that leaf.
Hidden and underscore-prefixed directories are skipped.
command.toml is optional and is an override/metadata escape hatch, not the primary source of command placement.

Examples:

commands/status/run.sh            -> status
commands/repo/sync/run.sh         -> repo sync
commands/repo/sync/help.md        -> help for repo sync

Examples:

doctor/git-clean/run.sh
doctor/binaries/run.sh

Discovery should follow the existing V2 agent-discovery pattern in internal/config/pack.go.
Discovered commands and doctor checks should flow through pack loading rather than being re-scanned later from raw pack dirs.
CLI command registration should use binding names, not pack names.
Existing script execution behavior should be preserved as much as possible.