[P0] Make just clean safe for tracked sources and documentation

[realmarcin]

Problem

The clean recipe in justfile removes src/communitymech/datamodel/*.py and every top-level docs/*.md file. The latter currently includes 19 tracked, hand-written guides. A routine cleanup command can therefore erase source and documentation that must be recovered from Git.

Priority

P0: this is a destructive local-development footgun and should be fixed before broader cleanup or documentation work.

Scope

• Replace broad globs with an explicit allowlist of disposable generated artifacts.
• Decide whether the generated datamodel belongs in clean; if it does, make regeneration explicit and safe.
• Add a regression test that resolves every target of the recipe and fails if any target is tracked by Git.
• Keep the recipe idempotent when optional generated outputs are absent.

Acceptance criteria

• just clean removes only documented generated artifacts.
• No path returned by git ls-files can be removed by the recipe.
• The 19 tracked docs/*.md guides and generated datamodel source remain intact.
• The regression test fails when a tracked path is added to the cleanup allowlist.
• just test, just lint, and git diff --check pass.