Add plan/draft mode for repo intelligence before coding (#29)

[jrchatruc]

Summary

• Adds an optional plan mode to the task pipeline: when enabled, Claude first analyzes the repo in read-only mode (all write tools blocked), produces a structured Markdown plan, and pauses for user approval before writing any code
• Users can approve to proceed to execution, reject to cancel with no changes, or send revision comments to iterate on the plan
• When plan mode is off (default), the pipeline runs exactly as before — no regression

Changes

Backend (dashboard/backend/src/):

• db.rs: Added plan_mode BOOLEAN NOT NULL DEFAULT false column migration
• models.rs: Added plan_mode to Task struct and CreateTaskRequest
• tasks.rs: New run_claude_plan_streaming() (read-only Claude), build_plan_prompt(), plan_approval_loop(), plan phase in run_task_pipeline(), updated all SELECT queries

Frontend (dashboard/frontend/src/):

• lib/api.ts: Added plan_mode to Task type and createTask params
• lib/status.ts: Added planning (spinner) and awaiting_plan_approval (blue/Clock) statuses
• pages/Tasks.tsx: "Generate plan before coding" checkbox in task creation form
• pages/TaskDetail.tsx: Plan statuses in CHAT_STATUSES, passes taskStatus to TaskChat
• components/TaskChat.tsx: Plan Review header, blue banner, Approve/Reject buttons

Test plan

• Create a task without plan mode → verify pipeline runs exactly as before
• Create a task with plan mode → verify status goes planning → awaiting_plan_approval
• Verify Claude's plan renders as Markdown in chat with Approve/Reject buttons
• Send a revision comment → verify Claude revises the plan (still read-only)
• Click "Approve Plan" → verify execution starts with plan context, normal push/preview flow
• Click "Reject" on a plan-mode task → verify task completes with no code changes
• Verify token usage includes both plan and execution runs

Closes #29