refactor(runtime): decompose session-manager.ts god-object (6445 lines, 40+ exports)

[0xMudit]

Summary

packages/runtime/src/session-manager.ts is 6,445 lines with 40+ exported interfaces, mixing session lifecycle, child agent spawning, agent graph intent, conversation copy, configuration transitions, plan state, and recovery. This god-object makes every change touch one file and makes ownership unclear.

Evidence

• packages/runtime/src/session-manager.ts:1-6775 — exports 8+ categories of types including SpawnChildAgentInput, ProvisionAgentGraphOperatorInput, PlanSafeBoundaryContinuationInput, SessionConfigurationTransitionRequest, CompactSessionInput
• runtime-host already consumes these via named sub-exports (@maka/runtime/session-manager); splitting internally would not break the public API
• Related files that already exist but are underused: session-projection-helpers.ts, session-recap.ts, session-title.ts, session-trace-projection.ts

Proposed Decomposition

Extract along existing seams into separate modules:

1. session-manager.ts — session CRUD, lifecycle, and configuration management
2. child-agent-manager.ts — child agent spawning, graph intent, and agent graph coordination
3. session-recovery.ts — recovery, continuation authority, and history repair
4. session-conversation.ts — conversation copy and transcript operations

Each extracted module would be consumed through the same barrel export path, preserving backward compatibility.

Acceptance Criteria

• session-manager.ts is under 2,000 lines
• Each extracted module has a clear single responsibility
• All existing tests pass without modification
• Public API surface (@maka/runtime/session-manager) is unchanged
• No new cross-module cycles introduced

Labels

refactor, help wanted

[spuru9]

take