Dev-manifest dependencies at a ref: read dependencies.rb from the pinned checkout

[JPDuchesne]

Context

gh/git dependencies are currently self-contained by contract: GhRepository/GitRepository claim Declarations::Resolved([]), meaning a source checkout's needs are declared by the consuming project's own dependencies.rb rows.

The Cargo comparison sharpened what changes when that contract ends: in Cargo, a git universe's version facts come from the manifest inside the checkout — the ref merely locates which checkout to read. When dev supports dev-manifest subprojects, the same shape applies here:

• find(id, probe: ref) reads the dependencies.rb at that ref (via gh api content fetch or a shallow clone) and reports its declarations as the version's Resolved claim — requirements, not the subproject's deps.lock (lock files are never availability facts; the sub-lock is at most a solver preference, Feature: prefer sub-lock pins as solver preferences (conservative resolution) #148).
• The subproject's declarations are already ScopedDeclarations; scope propagation semantics must be settled first (see the scope propagation issue).
• Constraint check on convergence must be loud (Resolver silently skips constraint checks on already-resolved transitive edges #147) before diamond deps across subprojects are trustworthy.

Acceptance

• gh/git versions whose repo carries a dependencies.rb report a real Resolved claim
• The self-contained contract remains for repos without a dev manifest
• Documented in the transitive regimes table