Agent pushes user toward implementation choices while still in plan mode

### Describe the bug

When working in plan mode (Shift+Tab / `[[PLAN]]` prefix), the agent frequently presents questions and choices that are implementation-oriented — pushing the user to commit to specific implementation decisions before they're ready to leave the planning phase.

For example, while still brainstorming and refining a plan, the agent might ask:

- `Should I start implementing X now?`
- `Which file should I create first?`
- `Ready to proceed? [Yes, start implementing / No]`

This breaks the planning workflow. The user is still iterating on the *what* and *why*, but the agent keeps steering toward the *how* and *when*.

### Expected behavior

While in plan mode, the agent should:

- **Stay in the planning domain** — ask questions about scope, priorities, trade-offs, and approach
- **Not offer to start implementing** or present implementation-action choices
- **Respect that the user may have more input** to contribute to the plan before transitioning
- **Wait for an explicit signal** from the user to begin implementation (e.g. `start`, `do it`, `implement it`, or switching out of plan mode)

Plan mode should feel like a collaborative *design* conversation, not a countdown to execution.

### Example of the problem

1. User (in plan mode): `I want to refactor the auth module`
2. Agent: outlines a plan, then immediately asks `Should I create the new auth service file now? [Yes / No]`
3. User: `Wait, I also want to consider moving the token logic...`
4. Agent: `Got it. Should I start with the token logic refactor? [Start now / Plan more]`

The agent keeps circling back to `let's start` rather than staying in the planning headspace.

### Proposed improvement

- When in plan mode, bias questions toward planning concerns (scope, dependencies, open questions, trade-offs)
- Suppress implementation-action choices (`start`, `create file`, `run command`) until the user explicitly transitions out of plan mode
- Treat plan mode as a hard boundary — the agent should never suggest or begin implementation while `[[PLAN]]` is active

### Additional context

This is especially disruptive during complex multi-phase plans where the user needs several rounds of back-and-forth to shape the approach before any code gets written. Being repeatedly nudged toward implementation breaks the flow and adds friction.