Add first-class git source groups for multi-repo docs configs

[KayleeWilliams]

Problem

Remote acquisition is first-class in Leadtype, but the config is collection-first: every collection carries repository, ref, and cacheDir, even when several collections come from the same repository.

The real public-docs configs show the resulting friction:

• c15t docs and changelog are separate collections from one Git repository;
• a shared object reduces repetition, but acquisition and content mapping are still conceptually mixed;
• production configs need custom code to normalize repository URLs and refs;
• sourceConfig ownership is declared at collection level even when it is a source-level policy;
• agents must infer that collections sharing (repository, ref) are cloned only once.

Leadtype already deduplicates acquisition internally. The API should expose the same mental model.

Proposal

Add an additive source-group helper that declares acquisition once and content collections beneath it. Exact naming should align with the canonical config work in #151; a possible shape is:

export default defineLeadtypeConfig({
  sources: {
    c15t: gitSource({
      repository: "https://github.com/c15t/c15t.git",
      ref: "main",
      cacheDir: ".leadtype/c15t",
      collections: {
        docs: {
          dir: "docs",
          routePrefix: "/docs",
          inheritConfig: true,
        },
        changelog: {
          dir: "changelog",
          routePrefix: "/changelog",
        },
      },
    }),
  },
});

The source should own:

• repository URL and revision;
• authentication/environment conventions;
• cache/sync policy;
• default config inheritance policy;
• one acquisition lifecycle shared by its child collections.

Each collection should own:

• content directory and include/exclude rules;
• public route prefix/mounts;
• navigation and schema overrides;
• collection-specific inheritance exceptions.

Compatibility and behavior

• Keep the existing flat collections map working and normalize both forms into the same source graph.
• Preserve current clone deduplication and offline behavior.
• Detect contradictory acquisition settings for collections that point at the same source.
• Make revision pinning visible: allow branch/tag/SHA, but surface an actionable warning for mutable production refs.
• Report source and collection IDs consistently in human and JSON output.
• A failed source acquisition should name every dependent collection.

Acceptance criteria

• One remote repository with multiple collections declares acquisition fields once.
• Existing flat collection configs remain supported.
• Both syntaxes resolve to the same internal source/collection graph.
• Sync performs one clone/fetch per resolved source and reports dependent collections.
• Source-level defaults can be overridden safely per collection.
• Conflicting settings fail before acquisition with an actionable message.
• Documentation includes local, single-remote, and multi-repo examples.
• The c15t public-docs config can adopt the helper with a meaningful reduction in boilerplate.

[pullfrog]

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[coderabbitai]

🔗 Related PRs

#31 - Support multiple docs source mounts [merged]
#53 - Add collections + sync for declarative multi-source docs [merged]
#56 - Add config-driven docs navigation [merged]
#59 - Restructure docs for agent-ready site workflow [merged]
#79 - Add source config inheritance for remote docs [merged]

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