Schema: milestones cannot participate in any relationship type except `precedes` with other milestones

[Mearman]

Problem

Milestone nodes are excluded from all relationship endpoint type combinations except precedes between milestones. This means milestones always form an isolated subgraph — they can only connect to each other, never to the rest of the graph.

Reproduction

sysprom add milestone --name "Phase 1"
sysprom add concept --name "Organisation"
sysprom add capability --name "AI-assisted account management"
sysprom add intent --name "Platform intent"

# All of these fail:
sysprom update add-rel MILE1 depends_on CON1   # Invalid
sysprom update add-rel MILE1 affects CAP1       # Invalid  
sysprom update add-rel MILE1 refines INT1       # Invalid
sysprom update add-rel CAP1 affects MILE1       # Invalid
sysprom update add-rel INT1 refines MILE1       # Invalid

Every relationship type rejects milestone as either source or target (except precedes between milestones/stages/gates).

Impact

Milestones always form a disconnected component in the graph. There is no way to express:

• "Phase 4 depends on the Organisation concept being adopted"
• "Phase 3 requires the Sandbox Execution capability"
• "Phase 1 refines the AI-assisted account management capability"

This makes milestones purely sequential markers with no provenance connections to the system they describe.

Suggestion

Add milestone to valid endpoint types for at least depends_on (source) and affects (target), so milestones can connect to the concepts, capabilities, and decisions they relate to.

Related

#22 — originally filed for depends_on specifically, but the problem is broader: milestones are excluded from all non-precedes relationships.