Pack v2: freeze commands and doctor final shape

[donbox]

Summary

We need to freeze the durable Pack v2 contract for commands/ and doctor/ without accidentally hardening still-open design questions.

Why

The first-pass validation suite can safely gate discovery conventions, but the richer command/doctor meta-model is still under-specified. We should settle the final shape explicitly rather than letting tests or partial implementations fossilize it.

Freeze now

• commands/<path>/run.sh as the default command entrypoint
• doctor/<path>/run.sh as the default doctor entrypoint
• directory shape implies command/check words by default
• help.md as the default adjacent help surface
• command.toml / doctor.toml as optional escape hatches
• imported command/doctor exposure through import bindings

Decide here

• what command.toml and doctor.toml are allowed to mean long-term
• how much symmetry we want between command and doctor manifest shapes
• collision policy boundaries
• whether there is any richer top-level extension / command-identity model beyond the default directory convention

Out of scope for the current suite gate

• final manifest symmetry/shape as a release gate
• broader design ambitions that are not yet implemented or frozen

References

• docs/packv2/doc-directory-conventions.md
• docs/packv2/doc-pack-v2.md
• docs/packv2/doc-conformance-matrix.md
• task: migration-validation-suite-first-pass