fix: let Plan tasks run in parallel without implicit auto-completion

[ASaiAnudeep]

Summary

Plan’s task list API always marked existing in-progress steps as complete whenever a new step was added. That made sense for simple sequential bots, but it blocked parallel work — even though the docs already showed a parallel pattern and per-task updates by ID were added earlier.

This PR adds an optional flag on task creation so callers can keep multiple steps in progress at once, while leaving the old sequential behavior as the default.

Opt-out flag, default on. We considered removing auto-completion entirely. That would’ve been cleaner for parallel use but would’ve broken existing sequential bots that rely on implicit “move to next step” behavior. Defaulting to the current behavior keeps upgrades safe; parallel callers pass the flag off.

No broader API redesign. Task completion stays explicit via status updates and the existing “complete plan” flow. The change is scoped to when a new task is appended.

Test plan

Unit tests

Checklist

• All commits are signed and verified
• pnpm validate passes
• Changeset added (or N/A — see CONTRIBUTING.md)
• Documentation updated (or N/A)

closes #630

[vercel]

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