Add five-minute parallel CI task contract

[Nickfost]

Closes #14.

What changed

• replaces the two-job fleet interface with named, deterministic task shards
• keeps fast and full as aggregate local/developer commands
• adds a project-owned scripts/ci/plan.json contract
• adds a composite planner action that validates the plan and emits a GitHub matrix
• enforces a five-minute hard job timeout and four-minute expected test payload
• expands the 45-test-minute reference suite into 13 independently schedulable jobs
• includes task and shard identity in Compose isolation to prevent concurrent collisions
• updates the configuration schema from version 1 to version 2
• updates migration, compliance, agent, and public-template documentation
• adds a copyable parallel GitHub Actions workflow

Safety

This is still an inert contract and example change. It does not register runners, alter a live runner group, migrate either existing project, add secrets, or deploy infrastructure.

Local validation

• 7 task-plan expansion tests pass
• 12 configuration policy tests pass
• fast plan expands to 6 jobs / 20 test-minutes
• full plan expands to 13 jobs / 45 test-minutes
• reference and multi-host configurations validate
• initializer produces a strict schema-v2 configuration
• shell and Python syntax checks pass

Operational meaning

With at least 13 available workers, the reference suite can schedule all 45 test-minutes at once while keeping each expected payload at four minutes or less. With fewer workers, GitHub queues the remaining shards without changing the project contract.