Migrate to the manifest-based the_local provider model

[tylercschneider]

Moves keystone_ui off the old TheLocal.register companion and onto the current manifest-based provider model, and clears out the dead code the migration surfaced.

the_local

Depends on the released the_local 0.4.0 (~> 0.4), the first published version carrying the manifest model.

The gem carries no Ruby for the_local anymore. Deleted KeystoneUi::Companion (lib/keystone_ui/the_local.rb), KeystoneUi::Reference, lib/keystone_ui/reference/guide.md, the three rendered locals under lib/, both requires (entrypoint + Rakefile), and companion_test.rb / reference_test.rb.

In their place: the_local/interface.yml declares the public surface — 2 install entry points (bin/rails generate keystone:install, KeystoneUi.configure) and 49 develop (all 48 ui_* helpers plus Keystone::Ui::Column) — and rake the_local:author rendered the_local/agents/keystone_ui-{info,install,develop}.md from it. rake the_local:check passes.

The manifest's sources: lists the component classes, not just the helper file: every ui_* helper is a **args pass-through, so the parameter contract only exists in each component's initialize.

Dead code removed

• keystone:inject_source / keystone:clean_source (lib/tasks/keystone_ui.rake) — they rewrite the /* keystone:source */ marker that install_generator.rb:49 calls "legacy" and strips on install. Nothing under ~/projects referenced them. This closes the open question in PLAN.md §5.
• KeystoneUi::Current — accent_override / surface_override referenced by nothing in the gem or in keystone_colors, which uses KeystoneUiHelper instead.
• bundle exec rspec in the docs — CLAUDE.md and CONTRIBUTING.md documented a test framework this repo doesn't use; the suite is Minitest.

Guards

• test/keystone_ui/packaging_test.rb asserts the committed locals ship — spec.files needed the_local/**/* or the gem contributes nothing. Verified red before the fix.
• A locals CI job runs rake the_local:check, so a manifest/local drift fails the build.

423 runs, 0 failures. Rubocop clean.

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