Improve output styling controls and document runtime behavior

[pasunboneleve]

Summary

• make hook output inherited and dimmed by default
• preserve dim styling across child ANSI color sequences and UTF-8 subprocess output
• add detailed behavior and configuration docs
• clarify AGENTS requirements for changelog updates on user-visible changes

Testing

• cargo fmt
• cargo test
• cargo clippy --all-targets --all-features -- -D warnings
• roborev review 18e42e4 --wait

Notes

• already contains the UTF-8 fix up to ; this branch layers the dimming, hook output, docs, and AGENTS updates on top.
• blog-repo tuning is being handled separately in the client config, not in this PR.