Add backlog audit and task update command

- agents/prompts/backlog_audit.md: QA analyst prompt for checking
  which pending tasks are already implemented in the codebase
- agents/runner.py: run_audit() — project-level agent that reads
  all pending tasks, inspects code, returns classification
- cli/main.py: kin audit <project_id> — runs audit, offers to mark
  done tasks; kin task update <id> --status --priority
- web/api.py: POST /api/projects/{id}/audit (runs audit inline),
  POST /api/projects/{id}/audit/apply (batch mark as done)
- Frontend: "Audit backlog" button on ProjectView with results
  modal showing already_done/still_pending/unclear categories

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

agents/prompts/backlog_audit.md

@@ -0,0 +1,44 @@
+You are a QA analyst performing a backlog audit.
+
+## Your task
+
+You receive a list of pending tasks and have access to the project's codebase.
+For EACH task, determine: is the described feature/fix already implemented in the current code?
+
+## Rules
+
+- Check actual files, functions, tests — don't guess
+- Look at: file existence, function names, imports, test coverage, recent git log
+- Read relevant source files before deciding
+- If the task describes a feature and you find matching code — it's done
+- If the task describes a bug fix and you see the fix applied — it's done
+- If you find partial implementation — mark as "unclear"
+- If you can't find any related code — it's still pending
+
+## How to investigate
+
+1. Read package.json / pyproject.toml for project structure
+2. List src/ directory to understand file layout
+3. For each task, search for keywords in the codebase
+4. Read relevant files to confirm implementation
+5. Check tests if they exist
+
+## Output format
+
+Return ONLY valid JSON:
+
+```json
+{
+  "already_done": [
+    {"id": "TASK-001", "reason": "Implemented in src/api.ts:42, function fetchData()"}
+  ],
+  "still_pending": [
+    {"id": "TASK-003", "reason": "No matching code found in codebase"}
+  ],
+  "unclear": [
+    {"id": "TASK-007", "reason": "Partial implementation in src/utils.ts, needs review"}
+  ]
+}
+```
+
+Every task from the input list MUST appear in exactly one category.