Add collaborator projection instructions

docs/source-models.md

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+# Source Models
+
+The projection system supports two source styles. They solve different social
+problems.
+
+## Git-Backed Shared Docs
+
+Use this for documents multiple collaborators should edit together:
+
+- project specs
+- game design docs
+- agendas
+- operating docs
+- shared research summaries
+
+The canonical source is a hosted Git repo in Gitea. Everyone clones, branches,
+commits, pulls, and pushes using normal developer tools. AFFiNE receives a
+read-only projection.
+
+Flow:
+
+```text
+Gitea repo
+  -> collaborator clone/edit/commit/push
+  -> projector checkout pull
+  -> projection registry
+  -> read-only AFFiNE docs
+```
+
+Why this is the default for shared work:
+
+- Git already solves collaborative text editing.
+- People can use their own editor, diff, review, and branch workflow.
+- File names and directory structure stay visible and portable.
+- AFFiNE stays useful as the shared view and whiteboard layer.
+
+## Mounted Personal Vaults
+
+Use this when a collaborator wants to share a selected slice of their own
+knowledge base without moving custody into the shared repo:
+
+- personal research notes
+- meeting notes
+- private vault excerpts
+- owned reference material
+
+The canonical source stays in the person's local vault. Projection is one-way
+into AFFiNE. Even the owning person should treat the AFFiNE copy as derived.
+
+Flow:
+
+```text
+personal Markdown or Obsidian vault
+  -> selected include paths
+  -> projection registry
+  -> read-only AFFiNE docs
+```
+
+Why this exists:
+
+- It preserves local custody.
+- It avoids forcing everyone into one giant shared repo.
+- It lets people publish useful slices without adopting a new editor.
+
+## Choosing Between Them
+
+Use Git-backed docs when the document itself is a shared artifact.
+
+Use mounted personal vaults when the document is personally owned but useful to
+surface in the collective workspace.
+
+If a personal note becomes a shared artifact, copy or adapt the useful content
+into a Git-backed repo and let that shared repo become canonical.