Add concurrent step execution with dependency-aware scheduler

[ubcent]

Motivation

• Improve task runtime by running independent steps in parallel instead of sequentially.
• Ensure correct dependency semantics so dependents wait for successful completion and fail fast on dependency failures.
• Provide clear, non-interleaved logging per service and make state updates safe under Promise-based concurrency.

Description

• Add a worker-pool scheduler runStepsConcurrently in src/lib/runner.ts that dispatches steps respecting depends_on, propagates dependency_failed, preserves input order in results, and honors a maxConcurrent limit.
• Replace the sequential loop in src/commands/start.ts with runStepsConcurrently(...), using a per-service scoped logger via logger.withPrefix('[service]') and updating step state with the new async updateStep(projectRoot, taskId, service, updates) API.
• Implement disk-safe concurrent state updates in src/lib/state.ts via updateStep(...) that re-reads state and writes atomically to .vexdo/state.json.tmp then renames.
• Add logger.withPrefix in src/lib/logger.ts, extend config/types with optional maxConcurrent and validation in src/lib/config.ts, and tighten task dependency validation (unknown deps, self-deps, cycles) in src/lib/tasks.ts.

Testing

• Ran the full test suite with npm test which includes unit and integration tests and all tests passed.
• Ran type checking with npm run typecheck and linting with npm run lint, both succeeded after fixes.
• Added unit tests for the scheduler in test/runner.test.ts and updated state/config tests to validate updateStep behavior and maxConcurrent parsing, which were executed as part of the test run and passed.

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Codex Task