Add semantic-checkpoint plan gate

[ictechgy]

Summary

• Add a plan-only/eval-only semantic-checkpoint experiment gate under context-guard experiments plan semantic-checkpoint.
• Emit deterministic readiness metadata with exact fallback validation, provenance review notes, protected-zone denial, and provider-boundary claim discipline.
• Sync the plugin executable copy and document the new gate across root/plugin/kit docs and the experimental radar.

Safety boundaries

• No emit, record, or serve runtime for semantic-checkpoint.
• No new binary.
• No file writes, transcript/prompt edits, model/provider/network calls, replacement context, or hosted token/cost savings claims.

Validation

• git diff --check
• python3 -m py_compile context-guard-kit/experimental_registry.py tests/test_context_guard_kit.py plugins/context-guard/bin/context-guard-experiments
• python3 scripts/sync_plugin_copies.py --check
• Targeted semantic unittest suite: Ran 7 tests ... OK
• Full suite: python3 -m unittest tests.test_context_guard_kit → Ran 611 tests in 247.860s — OK
• Source/plugin CLI smoke: semantic-checkpoint ready_for_plan_review [] None
• Architect verification: APPROVED