refactor(just)!: move to directory with flat recipes

[retr0h]

Summary

Converts the just module to the pattern established by md in #37: its own
directory with its own README, consumed via import? with flat prefixed
recipes instead of mod? with :: namespacing.

just/
├── just.just     recipes (just-fmt, just-fmt-check)
└── README.md     usage, recipes, requirements, exclusions
just.mod.just     removed

The :: namespacing bought nothing. Prefixed names read better at the call
site (just just-fmt-check), and dropping the shim removes the
working-directory indirection — the same indirection that made docs.mod.just
unusable in repos without a docs/ directory.

Breaking change

Consumers must change:

mod just '.just/remote/just.mod.just'
just just::fmt-check

to:

import? '.just/remote/just.just'
just just-fmt-check

and update the fetch URL to main/just/just.just, dropping the
just.mod.just fetch entirely.

Affected repos: gohai, osapi, nats-client, nats-server — each uses
mod just plus just just::fmt-check in just-lint.yml. These will fail
their Just Lint job until ported.

In-repo updates

• justfile now uses import 'just/just.just' and dogfoods the flat names
• .github/workflows/just-lint.yml runs just just-fmt-check directly,
  replacing just --justfile just.just --working-directory . fmt-check
• Root README gains a just row in the module table; the inline
  ### just.just section is removed
• CLAUDE.md Repository Structure updated

Testing

• just test (self-lint) passes — the repo formats its own justfiles through
  the converted module
• Verified the check still catches nested violations: deliberately corrupting
  just/just.just gives exit 1, restoring gives exit 0
• just/README.md is stable under md-fmt-check

Follow-up

Two modules now live in directories (md, just); go, bats, docs,
docker, and react remain at the root in mod? style and are still
documented inline in the root README.

🤖 Generated with Claude Code

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[retr0h]

Superseded — redone against current main. This branch predates the */.just/* exclusion added since, so merging it would regress the fix that stops the linter walking fetched modules.