feat: Phase 2 singleton state management (Issue #598)

[jlin53882]

Phase 2: Singleton State Management — Continuation of PR #430

Issue

Issue #598

Problem

memory-lancedb-pro calls register() multiple times per process lifecycle:

• 5× at startup (one per scope init)
• 4× per inbound message that triggers a scope cache-miss

Without guarding, all heavy plugin resources and session Maps are recreated on each register() call, causing:

1. Memory heap growth — accumulated Maps never released (~9× resource waste)
2. Duplicate handlers accumulating unboundedly (200+ observed)
3. Session Maps being lost when clearInternalHooks() resets the plugin between registrations

Solution: Phase 2 Singleton State

This PR implements Phase 2 of the multi-phase fix for Issue #430 / Issue #598.

Building on Phase 1 (hook event deduplication via _hookEventDedup Set, merged as PR #617), Phase 2 addresses resource deduplication:

1. PluginSingletonState interface — defines all 18 properties returned by the factory (8 heavy resources + 8 session Maps)
2. _singletonState module-level variable — null until first register() call, then persists for process lifetime
3. _initPluginState(api) factory function — creates all resources and Maps exactly once
4. if (!_singletonState) guard in register() — subsequent calls reuse the singleton via destructuring

interface PluginSingletonState {
  config, resolvedDbPath, store, embedder, decayEngine, tierManager,
  retriever, scopeManager, migrator, smartExtractor, extractionRateLimiter,
  // Session Maps — persist across scope refreshes
  reflectionErrorStateBySession, reflectionDerivedBySession, reflectionByAgentCache,
  recallHistory, turnCounter, autoCaptureSeenTextCount,
  autoCapturePendingIngressTexts, autoCaptureRecentTexts
}

let _singletonState: PluginSingletonState | null = null;

function _initPluginState(api: OpenClawPluginApi): PluginSingletonState {
  // ... create all resources and Maps exactly once
}

register(api: OpenClawPluginApi) {
  if (!_singletonState) { _singletonState = _initPluginState(api); }
  const { config, store, embedder, /* ... 16 more */ } = _singletonState;
  // register handlers using destructured resources
}

What Phase 3 Is NOT

Phase 3 (backup timer optimization) is NOT included in this PR.

Files Changed

• index.ts — singleton infrastructure (PluginSingletonState, _singletonState, _initPluginState, register() refactor)

Testing

• ✅ CLI smoke test passed (npm run test:cli-smoke)
• ✅ Functional e2e test passed
• ✅ All Maps appear exactly once in source (no duplicates)
• ✅ Braces balanced (1013/1013)
• ⚠️ test:core-regression has a pre-existing migrator is not defined error in upstream (16ee3e4) — not caused by this PR

Commits in This PR Branch

a147335 feat: Phase 2 singleton state management (Issue #598)
         (on top of upstream master 16ee3e4)