[FEATURE] Title: Feature request: /merge command — batch-integrate queued /btw notes at natural breakpoints

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Body:

Currently /btw queues a side note without interrupting. There's no way to batch-process those notes and integrate them
with in-progress work.

Proposed Solution

Proposed: /merge (or /sync)

At a natural stopping point, the user runs /merge. The agent:

1. Pulls all side notes queued via /btw since the last merge
2. Reads the current working context (files open, changes in flight, pending tasks)
3. Reasons about all of them together — surfaces conflicts, synergies, ordering dependencies
4. Produces a unified plan and executes it

Why this matters over just typing the idea normally:
A normal message forces an immediate context switch. /merge respects the current task's completion boundary while
ensuring side thoughts don't get lost. It's the difference between git stash + rebase and throwing a commit on top
mid-rebase.

---
That's specific enough to implement and explains the value prop clearly. Worth filing — /btw already exists so the
infrastructure for queuing is there.

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

CLI commands and flags

Use Case Example

Example workflow:
Working on a refactor. User /btw's three ideas during the session. At the end of the refactor, runs /merge. Agent
sees: idea 1 conflicts with a choice made in the refactor, idea 2 is already covered by it, idea 3 should go first
next session. Produces one coherent next-steps plan instead of four disconnected threads.

Additional Context

_No response_