Add deterministic release evidence bundle

[mdheller]

Summary

Adds the first ReleaseEvidenceBundle lane for Agent Machine.

Changes:

• Adds contracts/release-evidence-bundle.schema.json.
• Adds ReleaseEvidenceBundle schema mapping.
• Adds examples/release-evidence-bundle.bootstrap.json.
• Adds src/agent_machine/release_bundle.py to generate a deterministic, secret-free release evidence bundle from repository contents.
• Adds scripts/generate-release-evidence.py.
• Adds scripts/validate-release-bundle.py.
• Adds validate-release-bundle to make validate.
• Adds release bundle docs and links them from docs/index.md.
• Extends package import validation to include agent_machine.release_bundle.

Validation

Canonical validation remains:

make validate

Direct bundle generation:

python3 scripts/generate-release-evidence.py --pretty

Direct bundle validation:

python3 scripts/validate-release-bundle.py

Production note

This is still a bootstrap evidence lane. The bundle is not signed yet and does not prove production readiness. It gives us a deterministic payload that future signing, provenance, transparency, and release-promotion flows can use.