fix: stop writing generated_at to apm.lock.yaml (#2572)

[Aryan Singh K. (aryansk)]

Closes #2572

Summary

generated_at in apm.lock.yaml is a constant source of merge conflicts — two unrelated PRs that regenerate the lockfile collide only on this one line (reported in #2297 and again in #2572). This PR stops writing the key entirely, which is exactly what the triage suggested ("Open a PR that removes the generated_at field from apm-lock.yml generation").

Changes

• src/apm_cli/deps/lockfile.py: remove generated_at from LockFile.to_dict() output (the to_yaml write path). The field is still parsed from legacy lockfiles so existing projects load unchanged and apm lock export keeps its timestamp fallback.
• src/apm_cli/integration/mcp_integrator.py: drop the now-dead lockfile.generated_at = datetime.now(...) stamp before save; remove the unused import.
• tests/unit/install/test_mcp_lockfile_determinism.py: the determinism tests now assert generated_at is absent from the written YAML (the actual regression for [BUG] generated_at key in apm-lock.yml is constant source of merge conflicts. #2572) instead of asserting it round-trips.

Validation

• pytest tests/unit → 19,802 passed, 2 skipped, 21 xfailed
• ruff check + ruff format --check clean on all touched files

Compatibility

Legacy lockfiles that already contain generated_at keep parsing and the SBOM export fallback keeps working (it reads whatever the lockfile has). Newly written lockfiles simply omit the key.

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