observe: add opt-in content-free diagnostics #190

Goal

Opt-in, local-only Observe diagnostics for investigating high-retention results without collecting command arguments, paths, search patterns, URLs, or output contents.

Implemented

• Disabled by default; enable with CONTEXTCRAWLER_OBSERVE_DIAGNOSTICS=1.
• Writes bounded JSONL under the platform-local ContextCrawler data directory. Fixed-schema fields are allowlisted tool/filter labels, static outcome reasons, input/output byte and line counts, available grep match/group counts, and repeated non-empty line counts.
• Duplicate-line analysis samples at most 10,000 non-empty lines per input/output and marks sampled counts.
• Unix files/directories use private permissions; the log rotates at 5 MiB and retains one previous file.
• Diagnostics write failures warn on stderr but do not alter the Observe result.
• Documented the enable/disable, location, privacy boundary, sampling, and cleanup.

Validation and deployment

• Unit and real-binary tests verify the fixed schema, no content/path/command serialization, output identity with diagnostics enabled/disabled, write-failure behavior, permissions, rotation, and bounded sampling.
• Deployed from source to Brain and Pinky; verified the installed binaries, persistent opt-in, and live synthetic records on both hosts.
• cargo test --all-targets, cargo test (including doctests), cargo clippy --all-targets, rustfmt, and git diff --check passed.