Add default-off skill prompt dedup canary

[SecretSettler]

Summary

• Add the first runtime gray/canary replacement path for prompt dedup, default OFF
• Gate by env: CONTEXTPILOT_PROMPT_DEDUP_MODE=off|shadow|canary and kill switch CONTEXTPILOT_PROMPT_DEDUP_DISABLE=1
• Only mutates in canary mode, only same_type_skill_prompt_only exact duplicate blocks
• Keeps first occurrence verbatim; replaces later duplicates only if the reference is strictly shorter
• Never touches ordinary system, cross-type system/skill, user, assistant, or tool content
• Adds broad safety denylist for hard instruction/safety/security/tool/auth/secret/must/never/always/required/etc. blocks
• Adds metadata-only telemetry fields for realized prompt-dedup savings
• Documents canary usage and rollback

Verification

• /usr/local/lib/hermes-agent/venv/bin/python -m pytest tests/test_prompt_dedup_canary.py tests/test_hermes_plugin_patch.py -q
• /usr/local/lib/hermes-agent/venv/bin/python -m pytest tests/test_prompt_dedup_canary.py tests/test_hermes_plugin_patch.py tests/test_hermes_context_opportunity_analyzer.py tests/test_contextpilot_savings.py tests/test_contextpilot_savings_skill.py tests/test_hermes_contextpilot_monitor.py -q
• /usr/local/lib/hermes-agent/venv/bin/python scripts/analyze_hermes_context_opportunities.py --since-hours 24 --out-dir /tmp/contextpilot_canary_check --date canary_check --top-n 5
• /usr/local/lib/hermes-agent/venv/bin/python -m pytest -q
• Claude Code read-only review: PASS, no blockers

Full tests: 532 passed, 37 skipped, 3 warnings.
Stacked on PR #54.