docs(architecture): resolve one-way doors, clean up Phase 0 specs
Resolve blocking one-way door decisions:
- ADR-007: BiStream is a trait, handlers receive Connection not BiStream
- ADR-008: Secret service is CLI-embedded, exposed via call protocol
- ADR-009: One-way door decision framework (classify by reversal cost)
Update existing documents:
- overview.md: add design principles, revise ProtocolHandler signature,
update shared types, add WASM as design constraint
- open-questions.md: add door-type classifications, resolve OQ-01/OQ-08,
move OQ-09/OQ-10 to deferred section, mark two-way doors as impl-deferred
- README.md: reflect resolved questions, remove crate spec stubs from index
- ADR-002: cross-reference ADR-007 for signature revision
Clean up premature artifacts:
- Remove 11 empty crate spec stubs (16-28 lines each, no unique content)
- Specs will be created when each crate enters Phase 1