fix(repository): align with Composer's RepositoryCommand pipeline

Introduce JsonConfigSource in mozart-core mirroring Composer's
JsonConfigSource fallback logic (add/insert/set-url/remove repository),
and BaseConfigContext mirroring BaseConfigCommand's initialize().

Key behaviour fixes:
- list: synthesise [packagist.org] <disabled> only when no composer-type
  repo with a packagist.org host is present (was: always show enabled default)
- disable: idempotent via add_repository(false) matching Composer's branch;
  now requires a name (no silent default to packagist.org)
- enable: calls remove_repository only, no extra empty-array cleanup
- set-url: preserves assoc-keyed format instead of converting to list
- get-url: assoc fast-path + unquoted error message matching Composer
- add: use regex pre-check (starts_with '{') instead of trial-parse
- error messages reworded to match Composer verbatim (mozart brand kept)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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