Revert to v2.18.10 - Remove session persistence (v2.19.0-v2.19.5) (#322)

After 5 consecutive hotfix attempts, session persistence has proven
architecturally incompatible with the MCP SDK. Rolling back to last
known stable version.

## Removed
- 16 new files (session types, docs, tests, planning docs)
- 1,100+ lines of session persistence code
- Session restoration hooks and lifecycle events
- Retry policy and warm-start implementations

## Restored
- Stable v2.18.10 codebase
- Library export fields (from PR #310)
- All core MCP functionality

## Breaking Changes
- Session persistence APIs removed
- onSessionNotFound hook removed
- Session lifecycle events removed

This reverts commits fe13091 through 1d34ad8.
Restores commit 4566253 (v2.18.10, PR #310).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

src/index.ts

@@ -19,13 +19,6 @@ export {
   isInstanceContext
 } from './types/instance-context';
 
-// Session restoration types (v2.19.0)
-export type {
-  SessionRestoreHook,
-  SessionRestorationOptions,
-  SessionState
-} from './types/session-restoration';
-
 // Re-export MCP SDK types for convenience
 export type {
   Tool,