Command handler registry: core + migrate decider switch #136

What to build

Replace the monolithic switch (command.type) in decideOrchestrationCommand (apps/server/src/orchestration/decider.ts, 976 lines) with a CommandHandlerRegistry that dispatches by command type.

Registry design:

type CommandHandler = (command: OrchestrationCommand, readModel: OrchestrationReadModel) =>
  Effect<DecideOrchestrationCommandResult, OrchestrationCommandInvariantError | PlatformError>;

registry.register("thread.archive-and-new", handleArchiveAndNew)
registry.register("thread.context.trim", handleContextTrim)
registry.register("thread.context.compact", handleContextCompact)
registry.register("thread.context.summarize", handleContextSummarize)
registry.register("project.create", handleProjectCreate)
// ... etc

The decider becomes:

const handler = registry.get(command.type);
if (!handler) throw new Error(`No handler for command type: ${command.type}`);
return yield* handler(command, readModel);

Migrate these commands to individual handler files:

• All 4 h-code commands: archive-and-new, context.trim, context.compact, context.summarize
• At least 3 upstream commands (to prove the pattern works for both): e.g. project.create, thread.create, thread.delete

Each handler moves to its own file with its invariants co-located.

Acceptance criteria

• CommandHandlerRegistry built (reusing MethodRegistry pattern from Pre-factor: MethodRegistry generic utility #132)
• decideOrchestrationCommand replaced with registry dispatch
• All 4 h-code commands have dedicated handler files (e.g. handlers/archiveAndNew.ts)
• At least 3 upstream commands migrated to handler files
• Existing decider tests pass without modification (they call decideCommandSequence which delegates to registered handlers)
• decider.contextTrim.test.ts and decider.archiveAndNew.test.ts still pass
• vp check and vp run typecheck pass

Blocked by

• Pre-factor: MethodRegistry generic utility #132