docs(entity): entity docs describe data, not controller choreography (#350)

## Summary
### Why?

Entity doc comments had started narrating pipeline choreography — which
controller writes a field, which seams read it, which stage never
touches it (e.g. "written only by the orchestrator's speculate
controller … the seams never write it"). That is control-flow knowledge
that belongs with the code that owns the behavior; restated on the
entity it goes stale as the pipeline evolves and distracts from what the
entity actually guarantees.

### What?

Adds entity guideline 7 to CLAUDE.md: docs describe what a type or field
*is* and its invariants (immutability, uniqueness scope, units, valid
range) — never which controller/stage/seam reads or writes it; ownership
and write-path rules live in controller/store/extension docs. Lifecycle
enums may define states in terms of pipeline stages where that is the
state's meaning, but must not name the components performing
transitions. Also trims the one instance in the merged entity files this
stack touches (`SpeculationPathBuild.Version`'s "version arithmetic is
owned by the controller" clause — the convention is already documented
in CLAUDE.md and the storage README).

## Test Plan
Doc-only change.

## Issues


## Stack
1. @ #350
1. #351
1. #353

Author: behinddwalls

CLAUDE.md

@@ -120,6 +120,7 @@ Domain objects live under each domain's `entity/` tree, or under `platform/base/
 4. Every field must have a comment
 5. Reference other entities by ID (string or int), not directly
 6. String enums with sentinel values (`""` for unknown)
+7. Docs describe the data, not the choreography — say what a type or field *is* and its invariants (immutability, uniqueness scope, units, valid range), never which controller/stage/seam reads or writes it. Ownership and write-path rules live with the code that owns them (controller, store, or extension docs). Lifecycle enums may define states in terms of pipeline stages where that *is* the state's meaning (e.g. "admitted under the build budget"), but must not name the components that perform transitions.
 
 ### Extensions
 

submitqueue/entity/speculation_path_build.go

@@ -33,7 +33,7 @@ type SpeculationPathBuild struct {
 	CreatedAt int64
 	// Version is the version of the object. It is used for optimistic locking:
 	// updates are conditional on the persisted version matching the caller's
-	// expected version. Versioning starts at 1; version arithmetic is owned by
-	// the controller, the store performs a pure conditional write.
+	// expected version. Versioning starts at 1 and is incremented for each
+	// change to the object.
 	Version int32
 }