docs: propose role-scoped memory with internal scope model

[MatrixAdventurer]

What type of PR is this?

• feat (new feature)
• fix (bug fix)
• docs (documentation)
• style (formatting, no code change)
• refactor (code change that neither fixes a bug nor adds a feature)
• perf (performance improvement)
• test (adding or updating tests)
• chore (maintenance, tooling)
• build / ci (build or CI changes)

Which issue(s) this PR fixes

Related to #126

What this PR does / why we need it

This PR adds a design note for role-scoped memory spaces with inheritance.

The main idea is to separate:

• branch/snapshot for rollback, experimentation, and version control
• role scope for collaboration, inherited shared memory, and role-local overlays

The document proposes a model where clients can create a role like:

create role "writer" based on main

so recalls can use:

• shared base memory from main
• role-specific delta memory from writer

without encoding role semantics in raw content or fragmenting memory into separate user IDs.

It also sketches a minimum API / CLI surface for future implementation.

Local validation

cargo test -p memoria-api --test api_e2e test_remote_store_retrieve -- --nocapture

[MatrixAdventurer]

Closing this in favor of a cleaner future consolidation. This proposal overlaps with other role-scope drafts and has been stale for a while, so leaving it open adds confusion more than signal.