## Why

The kanban sync is currently implemented as a detailed workflow document that the LLM interprets and executes command-by-command. Each sync burns significant tokens as the LLM reasons through ~10 phases and executes ~15-30 `pcli` commands. The sync logic is entirely deterministic — there are no decisions that require LLM judgment. Moving this to a standalone bash script eliminates the overhead and makes sync near-instant. Additionally, there is no concurrency protection — two syncs could run simultaneously and clash on Planka API state.

## What Changes

- New `kanban-project-sync` bash script (deployed externally, on PATH) that performs the full 10-phase Planka board reconciliation
- Script accepts `--project` and `--board` as required inputs, with optional `--background` flag
- Script assumes it is run from the project root (uses `pwd` to locate `openspec/` directory)
- Implements `flock`-based concurrency control with a coalescing pending flag (depth-1 queue)
- Default mode is foreground (human-friendly); `--background` detaches for fire-and-forget use
- `.claude/commands/kanban-sync.md` simplified to just invoke the script with `--background`
- `CLAUDE.md` sync instructions updated to reference the script

## Capabilities

### New Capabilities
- `kanban-sync-concurrency`: Concurrency control for sync execution — flock-based locking with a pending flag that coalesces multiple requests, and automatic re-run after completion if a sync was requested during execution

### Modified Capabilities
_(none — no existing spec-level requirements change)_

## Impact

- **New external script**: `kanban-project-sync` — deployed to developer instances via existing tooling, placed on PATH
- **Modified**: `.claude/commands/kanban-sync.md` — reduced from full workflow to script invocation
- **Modified**: `CLAUDE.md` — updated Planka Sync section
- **Dependencies**: Requires `flock` (standard on Linux, available on macOS via `brew install util-linux`), `jq`, `yq`, `pcli`, `openspec` on PATH
- **No Go code changes** — this is entirely in the tooling/workflow layer
- **Convention**: Script must be run from the project root directory (same as `openspec` and `pcli`)