feat: inject concurrent agent count into persona system prompt

[nextlevelshit]

Summary

• Adds max_concurrent_agents field to pipeline step configuration with validation (1-10 range)
• Injects a concurrency directive into the persona's generated CLAUDE.md when the value exceeds 1
• Carries the concurrency setting through to AdapterRunConfig for adapter-level awareness
• Includes spec, plan, and tasks documentation for the feature

Closes #112

Changes

• internal/pipeline/types.go — Added MaxConcurrentAgents field to Step struct with YAML/JSON tags
• internal/pipeline/validation.go — Added validation for max_concurrent_agents range (1-10)
• internal/pipeline/validation_test.go — Table-driven tests for concurrency field validation
• internal/pipeline/executor.go — Passes MaxConcurrentAgents to adapter run config
• internal/adapter/adapter.go — Added MaxConcurrentAgents to AdapterRunConfig
• internal/adapter/claude.go — Injects concurrency section into generated CLAUDE.md between contract compliance and restrictions
• internal/adapter/claude_test.go — Tests for prompt generation with and without concurrency hints
• specs/112-concurrent-agent-prompt/ — Feature spec, plan, and task list

Test Plan

• Unit tests verify CLAUDE.md prompt generation includes concurrency directive when max_concurrent_agents > 1
• Unit tests verify no concurrency section when value is 0 or 1
• Validation tests confirm rejection of values outside 1-10 range and acceptance of valid values
• Integration test verifies the concurrency hint appears in adapter invocation config
• All tests pass via go test ./...