Schema: `milestone` excluded from `depends_on` source types

[Mearman]

Problem

Milestones cannot be the source of a depends_on relationship. The only relationship type available between milestones is precedes, which captures sequencing but not conceptual dependencies.

Reproduction

sysprom add milestone --name "Phase 4: Multi-channel expansion"
sysprom add concept --name "Organisation"
sysprom update add-rel MILE4 depends_on CON12

Error:

Invalid endpoint types for depends_on: milestone → concept.
Valid: [intent, concept, capability, element, realisation, invariant, principle, policy, protocol, stage, role, gate, mode, artefact, decision, change, view] → [...]

Use case

A product roadmap milestone may depend on a concept being defined or a decision being made before it can proceed. For example, "Phase 4: Multi-channel expansion" depends on the Organisation model and the Platform Abstraction Layer being in place. precedes only works between milestones, so there is no way to express this cross-type dependency.

Suggestion

Add milestone to the valid source types for depends_on.