Add structured cleanup outputs and validate them in CI

[justinthelaw]

Motivation

• Expose machine-readable cleanup results so downstream workflow steps can branch on reclaimed space, include cleanup summaries in job artifacts, and validate cleanup planning without parsing logs.

Description

• Add composite action outputs in action.yml: before-available-bytes, after-available-bytes, reclaimed-bytes, cleanup-profile, and executed-components and wire them to a new structured-outputs step.
• Capture / available bytes in the BEFORE and AFTER disk-report steps and publish them to $GITHUB_OUTPUT, and compute executed-components in the cleanup-plan step for emission.
• Introduce a final step that computes reclaimed_bytes (after - before) and emits all structured outputs consistently for the composite action to expose.
• Update documentation and tests by adding an Outputs reference and example to README.md, updating docs/MIGRATIONS.md and AGENTS.md, and extending .github/workflows/test.yml with an output-contract matrix job that validates output presence, numeric contracts, profile correctness, component inclusion, and reclaimed-bytes arithmetic.

Testing

• Validated the modified YAML files by running ruby -e 'require "yaml"; YAML.load_file("action.yml"); YAML.load_file(".github/workflows/test.yml"); puts "ok"', which printed ok.
• Attempted yamllint action.yml .github/workflows/test.yml but yamllint was not available in the environment, so a linter run was not performed here.
• Added CI coverage via the new output-contract workflow job, which will run on CI to verify outputs for both custom and max scenarios.

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Codex Task