chore: initial scaffolding for SynthesisAgent.OpenclawPlugin

OpenClaw plugin that manages long-lived interactive Claude Code processes
per OpenClaw session. Process manager + session-mapping persistence +
bridge WebSocket server skeleton.

Will be rewritten to follow the contractor-agent HTTP model-provider pattern
(register as `synthesis-claude-bridge` provider, receive /v1/chat/completions,
dispatch to channel notification on the bound Claude process). See parent
repo's STATUS.md for the punch list.

core/session-mapping.ts

@@ -0,0 +1,83 @@
+import { readFileSync, writeFileSync, mkdirSync } from 'node:fs'
+import { dirname } from 'node:path'
+import { randomUUID } from 'node:crypto'
+
+export interface SessionRecord {
+  openclawSessionId: string
+  claudeSessionUuid: string
+  createdAt: number
+  lastActiveAt: number
+}
+
+/**
+ * Persistent openclaw_session ↔ claude_session mapping.
+ *
+ * Single-writer (the OpenclawPlugin process). File-level on every mutation —
+ * simple and correct for the volume we expect. If it ever matters, swap for
+ * sqlite.
+ */
+export class SessionMapping {
+  private records = new Map<string, SessionRecord>()
+  private path: string
+
+  constructor(path: string) {
+    this.path = path
+    this.load()
+  }
+
+  private load(): void {
+    try {
+      const raw = readFileSync(this.path, 'utf8')
+      const parsed = JSON.parse(raw) as { records?: SessionRecord[] }
+      for (const r of parsed.records ?? []) {
+        this.records.set(r.openclawSessionId, r)
+      }
+    } catch {
+      // File doesn't exist or unreadable — start fresh.
+    }
+  }
+
+  private flush(): void {
+    mkdirSync(dirname(this.path), { recursive: true })
+    writeFileSync(
+      this.path,
+      JSON.stringify({ records: [...this.records.values()] }, null, 2),
+      'utf8',
+    )
+  }
+
+  get(openclawSessionId: string): SessionRecord | undefined {
+    return this.records.get(openclawSessionId)
+  }
+
+  /** Get-or-create. Returns the (possibly freshly assigned) claude_session_uuid. */
+  ensure(openclawSessionId: string): SessionRecord {
+    const existing = this.records.get(openclawSessionId)
+    if (existing) return existing
+    const now = Date.now()
+    const rec: SessionRecord = {
+      openclawSessionId,
+      claudeSessionUuid: randomUUID(),
+      createdAt: now,
+      lastActiveAt: now,
+    }
+    this.records.set(openclawSessionId, rec)
+    this.flush()
+    return rec
+  }
+
+  touch(openclawSessionId: string): void {
+    const r = this.records.get(openclawSessionId)
+    if (!r) return
+    r.lastActiveAt = Date.now()
+    this.flush()
+  }
+
+  forget(openclawSessionId: string): void {
+    if (this.records.delete(openclawSessionId)) this.flush()
+  }
+
+  all(): SessionRecord[] {
+    return [...this.records.values()]
+  }
+}