feat(cli): persist Claude session mappings across restarts

[2yclaw]

Summary

• Persist CLI agent conversationID → sessionID mappings to ~/.weclaw/sessions.json
• On startup, load saved sessions so --resume works across weclaw restarts
• On session update or reset, write back to disk immediately

Problem

Claude CLI sessions are stored on disk by Claude Code (~/.claude/projects/), but weclaw's in-memory sessions map[string]string is lost on restart. After weclaw restart, users start a new conversation even though the Claude session still exists — losing all context.

Solution

Add loadSessions() / saveSessions() to CLIAgent with these properties:

• File format: map[agentName]map[conversationID]sessionID — supports multiple CLI agents
• Write strategy: write-through on every mutation (2 call sites), file is tiny (~100 bytes per session)
• Atomic write: temp file + os.Rename to prevent corruption on crash
• Corrupt file handling: refuses to overwrite if existing JSON is malformed
• Graceful degradation: if persistence fails, logs warning and falls back to current behavior (new session)
• Stale sessions: Claude handles stale --resume IDs gracefully (auto-creates new session), so no TTL needed

Only affects CLI agents that use --resume (currently Claude). ACP and HTTP agents are unchanged.

Changes

• agent/cli_agent.go: +86 lines, 1 file changed

Test plan

• go build ./... — compiles cleanly
• go vet ./... — no warnings
• Manual test: send message → verify ~/.weclaw/sessions.json created
• Manual test: weclaw restart → send message → verify log shows resuming session with same session ID
• Manual test: weclaw restart → verify loaded N session(s) for claude in log