docs: define Forge Action dependency admission boundary

[pcvantol]

Summary

Clarifies the cross-product boundary for Forge-planned Engineering Action dependencies.

• Forge owns Action existence, target repository/write scope, hard depends_on decisions and predecessor evidence requirements.
• EP persists and fail-closed enforces the immutable submitted dependency snapshot; EP does not invent Forge planning dependencies.
• Hard logical dependency, repository/resource lease exclusion, and qualified Agent/provider capacity are three distinct gates and evidence states.
• Cross-repository predecessors can require concrete artifact evidence (artifact identity/digest, source revision, qualification/provenance), not merely source merge status.
• Independent repository Actions may be simultaneously eligible when the producer graph permits; EP still owns actual admission/placement/capacity.
• Updates the EP/Forge Platform boundary so final Forge Platform manifest work depends on published qualified EP Server/Project-Agent artifacts when Forge plans that edge.

Sequencing

The first Forge dynamic inner-Mission canary may remain serial and does not require full multi-repository parallel qualification. The later cross-repository canary requires this boundary plus EP multi-execution, repository lease and Agent/provider capacity qualification.

Boundary

Documentation/contract only. This PR does not modify or compete with the concurrent EP run-quality-assurance implementation PR #100.