feat(protocols): add cross-project boundary and signature image rules

Cross-Project Boundary tells the session to stop and ask before acting on a file or task scoped to a different project's .ai/, then write a handoff file at the target project's inbox/ if confirmed.

Signature Image documents the location of the transparent signature PNGs in the archsetup stow tree and limits stamping to explicit per-document requests — never signing on initiative.

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