feat(phase2): Singleton State Management (PR #598, 4 commits) (#637)

* feat(phase2): add PluginSingletonState interface and singleton state variable (PR #598)

- Add PluginSingletonState interface: defines all 18 properties returned by the
  singleton factory (8 heavy resources + 8 session Maps with their types)
- Add module-level _singletonState: null until first register() call
- Add _initPluginState(api) factory skeleton: returns PluginSingletonState

This commit only adds the interface and skeleton.
The factory body is empty (returns null!) and will be filled in subsequent commits.

* feat(phase2): replace register() init block with singleton factory call (PR #598)

- Replace the old init block (config, store, embedder, tierManager, retriever,
  scopeManager, migrator, smartExtractor, extractionRateLimiter) with:
  - if (!_singletonState) guard that calls _initPluginState()
  - destructuring of all 18 properties from _singletonState
- The factory _initPluginState() body is still empty (returns null!) —
  filled in the next commit
- Helpers section is unchanged but will reference undefined values
  until the factory body is added in the next commit (expected state)

* feat(phase2): fill _initPluginState() factory body with all resources and Maps (PR #598)

* feat(phase2): remove old duplicate session Map declarations from helpers (PR #598)

The helpers section had 8 Map declarations that are now created once in
_singletonState and accessed via destructuring. Remove them to prevent
duplicate declarations that would cause TypeScript errors.

Maps removed from helpers:
- reflectionErrorStateBySession
- reflectionDerivedBySession
- reflectionByAgentCache
- recallHistory
- turnCounter
- autoCaptureSeenTextCount
- autoCapturePendingIngressTexts
- autoCaptureRecentTexts

* fix(phase2): add missing migrator to singleton destructuring

migrator was returned by _initPluginState() but missing from the
singleton destructuring, causing ReferenceError at createMemoryCLI()
call site (index.ts:2286).

This was the root cause of CI failures in PR #598.

Author: jlin53882