feat: Add healthchecks and tool subcommand

[ogrman]

Summary

• Healthchecks for tasks: Tasks with run action can now specify a healthcheck. Dependent tasks wait for the healthcheck to pass before starting, similar to how ensure tasks block dependents until complete.
• Built-in healthcheck tools: Three tools available: is-port-open <port>, http-get <port or URL>, and http-get-ok <port or URL>. Using tool is more efficient than cmd since it doesn't spawn a new process for each check.
• rote tool subcommand: Run built-in tools directly from the CLI (e.g., rote tool is-port-open 5432). Useful for scripting and debugging.

Example usage

tasks:
  postgres:
    run: docker run --rm -p 5432:5432 postgres
    healthcheck:
      tool: is-port-open 5432
      interval: 0.5

  api:
    run: ./server
    require: [postgres]  # Won't start until postgres healthcheck passes

Test plan

• Unit tests for healthcheck config parsing
• Integration tests for healthcheck functionality
• Tests for rote tool subcommand
• Updated README and example.yaml

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