docs(#634): PLATFORM.md routing strategy convention + garden protocol

[mdproctor]

Context

engine#634 established the universal routing strategy convention. Documentation needs updating:

1. PLATFORM.md capability ownership table — new entry for Routing Strategy Resolution (casehub-platform-api)
2. PLATFORM.md Step 4 rules — new routing strategy rule
3. Garden protocol routing-strategy-convention.md — scope, rule, non-members

See engine#634 spec §7 for exact content.

Scale

S

Complexity

Low

[mdproctor]

PLATFORM.md updated (casehubio/parent), garden protocol created and indexed.