feat(agent): persist native session state in workspace snapshots

[tatoalo]

Problem

Directory resume snapshots preserve /tmp/workspace, while Claude Code and Codex keep their native session state under home-directory config paths. Snapshot resumes therefore lose that native state and fall back to rebuilding history from persisted run logs. That loses in-session compaction, starts resumes with unnecessarily full context, and prevents Codex from using native thread/resume.

Follow-up to #3331.

Changes

• Add optional POSTHOG_AGENT_STATE_DIR configuration to the agent server.
• Before session initialization, create durable state directories under that root and symlink the native session-bearing directories:
  • Claude Code: projects, session-env, plans, and todos.
  • Codex: sessions and shell_snapshots.
• Keep credentials, runtime configuration, installed skills, caches, and other ephemeral state outside the persisted root so resumed sandboxes receive current configuration.
• Leave existing behavior unchanged unless the sandbox launcher configures the state root. Directory-snapshot launchers can point it at /tmp/workspace/.posthog/agent-state so the existing workspace snapshot captures both agents' native resume state.

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 3045c06.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "feat(agent): persist native session stat..." | Re-trigger Greptile}

[github-actions]

Well-tested feature addition that safely migrates and symlinks native agent session directories into a durable workspace root. Both bot-flagged concerns (data loss on first opt-in, partial symlink state from parallel execution) are addressed in the current diff, and the migration logic is defensive against data loss. No API contract changes, no security issues.