feat(sessions): opt-in clear history and continue from plan

[igennova]

Summary

• Adds an opt-in plan-approval option: Approve, clear history, and continue from plan (alongside normal approve + reject).
• Normal Approve and proceed keeps planning history unchanged.

Why

Issue PostHog/posthog#76316: after planning, clearing context before implementation avoids auto-compaction mid-implementation. This mirrors Claude’s clear-on-plan-accept flow as an explicit choice.

How it works

1. User is in plan mode → agent proposes a plan
2. Plan card offers:
   • Approve and proceed (keep context)
   • Approve, clear history, and continue from plan (fresh context)
   • Reject with feedback
3. Clear path: resolve permission UI locally → cancelPrompt → createNewLocalSession with plan seed
4. Same task / title; new task run (chat thread resets by design)

Video

Screen.Recording.2026-07-15.at.12.49.28.PM.mov

Test plan

• Enter plan mode → create a plan → confirm new clear option appears
• Approve and proceed → planning history remains; agent continues in-place
• Clear history and continue → chat resets; agent starts from plan only; no ACP/error toast
• Mode selector still applies on clear path (e.g. auto / acceptEdits)
• Reject with feedback still works
• Error-state New Session still reconnects in place (resetSession unchanged)
• Unit tests: planContinuation, sessionServicePlanContinuation, permission-options, PlanApprovalSelector

Replaces #3462 , which was closed during the pre-migration PR sweep (not actually stale).

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[charlesvien]

Hey @igennova, we've migrated this repo into the PostHog/posthog monorepo, so this PR needs to be recreated there.

Easiest path: check out the monorepo and run the /porting-code-prs skill with your agent pointed at this PR. It has the migration guide and path remappings baked in so the agent doesn't have to figure them out. You don't strictly need it though, just asking your agent to port the PR works fine too.

Shout if you get stuck.