Cleanup pass for dead code and docs

[jssblck]

Why this change

• Nudge had accumulated exploratory surfaces that were no longer part of the supported product path: the benchmark package, the nudge claude run command, and the old nudge::claude agent-runner module. Keeping them made the workspace, docs, tests, and dependency graph harder to understand.
• This PR intentionally deletes those dead surfaces instead of preserving compatibility. Users or local scripts that still call nudge claude run, import nudge::claude, or depend on packages/benchmark will need to stop using those deleted paths.
• With the dead code gone, the remaining Nudge package is focused on hook adapters, rule loading/evaluation, setup, validation, and nudge check.
• The branch also cleans up the code that remains: rule schema matching is split into focused modules, the check command is decomposed into smaller scan and issue-building steps, and Claude/Codex setup now shares JSON hook merging logic.
• Documentation and integration tests were updated to match the leaner structure and the current Claude/Codex hook behavior.

Testing steps performed

$ cargo fmt --all
Warning: can't set `wrap_comments = true`, unstable features are only available in nightly channel.
Warning: can't set `wrap_comments = true`, unstable features are only available in nightly channel.
Warning: can't set `wrap_comments = true`, unstable features are only available in nightly channel.
Warning: can't set `wrap_comments = true`, unstable features are only available in nightly channel.

$ cargo test -p nudge
test result: ok. 59 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.02s
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
test result: ok. 70 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.76s
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

$ cargo clippy -p nudge --all-targets --all-features -- -D warnings
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.63s

Surprising changes / Notes

• This PR removes APIs and commands rather than leaving compatibility shims. That is deliberate cleanup, not an accidental omission.
• No runtime configuration changes are required for supported Nudge hook usage after merge.