feat(code): Add "clear and continue from plan" option in plan mode

[littlekirkycode]

Base branch: feat/clear-command — depends on resetSession()
with clearHistory

Closes #704

Summary

Adds a third option when exiting plan mode: "Yes, clear history and
continue from plan". Clears the conversation and re-sends the approved
plan as the first prompt — useful when the context window filled up
during planning and you want the agent to start implementing with maximum
available context.

Note: /clear resets the conversation by reconnecting with empty
history — previous messages are no longer visible in the UI. The session
log on S3 is not deleted, but the UI won't replay it.
That way if the you wants a softer version (e.g. keep history scrollable
but start a fresh context), please bear this in mind.

Flow

User exits plan mode
  → Permission dialog shows 3 options:
    1. "Yes, and auto-accept edits" (existing)
    2. "Yes, and manually approve edits" (existing)
    3. "Yes, clear history and continue from plan" ← NEW
  → Agent switches to default mode
  → Sends _posthog/clear_and_continue notification with plan text
  → Renderer queues plan in pendingPlanContinuations
  → After current turn completes:
    1. resetSession() clears history
    2. sendPrompt() re-injects the plan

Agent-side

The permission handler extracts the plan text and signals the renderer
via extNotification:

if (response.outcome.optionId === "clearAndContinue") {
  const planText = extractPlanText(updatedInput);
  // Switch to default mode, then signal renderer
  await context.client.extNotification(
    POSTHOG_NOTIFICATIONS.CLEAR_AND_CONTINUE,
    { sessionId: context.sessionId, plan: planText },
  );
}

Renderer-side

SessionService detects the notification, queues the plan, and executes
after the turn completes:

private async executeClearAndContinue(taskId: string, repoPath: string,
plan: string): Promise<void> {
  await this.resetSession(taskId, repoPath);
  await this.sendPrompt(taskId, `Continue implementing this approved
plan:\n\n${plan}`);
}

The plan is queued (not executed immediately) because the notification
arrives mid-turn — we wait for the current turn to finish before
clearing.

Changes

File	What
acp-extensions.ts	New CLEAR_AND_CONTINUE notification constant
permission-options.ts	Add clearAndContinue option to plan exit dialog
permission-handlers.ts	Handle clearAndContinue — switch mode, send notification
package.json	Export ./acp-extensions subpath
tsup.config.ts	Add acp-extensions.ts build entry
service.ts	pendingPlanContinuations map, notification detection, executeClearAndContinue()

Test plan

• Enter plan mode → create a plan → exit → new option appears
• Selecting it clears history, switches to default mode, sends plan
  as first prompt
• Agent continues implementing the plan in fresh context
• Existing "auto-accept" and "manually approve" options still work
  unchanged