feat: add quick answer command

[Microck]

Summary

• add kagi quick for Kagi Quick Answer via the authenticated web-product stream
• support json, compact, pretty, and markdown output
• normalize full Session Link URLs from env and config so KAGI_SESSION_TOKEN='https://kagi.com/search?token=...' works as documented

Implementation

• add the quick transport, tagged-stream parser, JSON types, and CLI wiring
• reuse shared lens validation for kagi quick --lens <INDEX>
• improve pretty rendering by using answer markdown and stripping inline footnote markers
• remove the unused legacy search() wrapper in src/search.rs

Docs and Demo

• add the quick command reference and update README, quickstart, auth matrix, output contract, coverage, error reference, docs nav, and llms index
• add scripts/demo-quick.sh
• record and ship docs/demo-assets/quick.gif and images/demos/quick.gif

Verification

• cargo fmt --all --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test
• cargo test quick::tests::live_ -- --ignored --nocapture
• cargo run -- auth check
• manual CLI checks for json, compact, pretty, markdown, --lens 0, invalid lens rejection, empty query rejection, and the demo preview flow

Notes

• kagi quick is single-turn only in this PR
• this feature depends on Kagi's live Quick Answer web-product stream, so upstream contract drift remains the main long-term risk