feat: oracle cross-validation agent with multi-backend retrieval

Add three scripts for cross-validating oracle ground truth:

- context_retrieval_agent.py: Claude-powered retrieval agent with 3
  backends (local/deepsearch/hybrid). Uses configurable model
  (default: Sonnet 4.6). Local tools: grep, rg, find, file reading,
  import search, symbol search. SG tools: deep search, keyword search.
  Cross-validate mode runs all backends and reports agreement metrics.

- validate_on_contextbench.py: Validates our agent against ContextBench
  (1,136 human-annotated SWE-bench tasks). Downloads dataset from
  HuggingFace, converts agent output to trajectory format, evaluates
  file/symbol/span recall and precision.

- cross_validate_oracles.py: Compares agent-generated oracles against
  existing SG oracles (oracle_answer.json) and hand-curated ground
  truth (ground_truth.json). Reports file F1, Cohen's kappa, per-suite
  breakdown, and divergence analysis.

Addresses circularity concern: current oracles are 100% Sourcegraph-
generated. Cross-validation with independent local tools empirically
tests whether SG and non-SG methods agree. Also addresses SDLC coverage
gap (69/150 tasks missing ground_truth.json).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sjarmak

docs/ops/SCRIPT_INDEX.md

@@ -140,6 +140,7 @@ Generated from `scripts/registry.json` by `scripts/generate_script_index.py`.
 ## Validation
 
 - `scripts/validate_enterprise_readiness.py` - Validation script for validate enterprise readiness.
+- `scripts/validate_on_contextbench.py` - Validation script for validate on contextbench.
 
 ## Generation
 
@@ -167,7 +168,9 @@ Generated from `scripts/registry.json` by `scripts/generate_script_index.py`.
 - `scripts/backfill_triage_from_manifest.py` [one_off] - Historical one-off script: backfill triage from manifest.
 - `scripts/check_harness_readiness.py` - Utility script for check harness readiness.
 - `scripts/compute_bootstrap_cis.py` - Utility script for compute bootstrap cis.
+- `scripts/context_retrieval_agent.py` - Utility script for context retrieval agent.
 - `scripts/control_plane.py` - Utility script for control plane.
+- `scripts/cross_validate_oracles.py` - Utility script for cross validate oracles.
 - `scripts/daytona_poc_runner.py` - Utility script for daytona poc runner.
 - `scripts/daytona_runner.py` - Utility script for daytona runner.
 - `scripts/dependeval_eval_dr.py` - Utility script for dependeval eval dr.