feat(assistant): add ask-page command

[Microck]

Summary

• add kagi ask-page <URL> <QUESTION> for page-focused Assistant queries
• return JSON with source metadata, thread metadata, and the Assistant reply
• refresh docs and add a recorded ask-page demo

Implementation

• reverse engineer the live web flow and confirm it reuses POST /assistant/prompt
• shape the prompt as URL + newline + question to match Kagi's page-aware Assistant behavior
• add URL and question validation plus a dedicated ask-page response envelope
• normalize full Session Link URLs when loading session auth from env or config

Docs and Demo

• add docs/commands/ask-page.mdx
• update README, quickstart, auth matrix, coverage, output contract, error reference, docs nav, and llms index
• add scripts/demo-ask-page.sh
• record docs/demo-assets/ask-page.gif and images/demos/ask-page.gif

Verification

• cargo fmt --all --check
• cargo test
• cargo test live_ask_page_rust_homepage -- --ignored --nocapture
• cargo clippy --all-targets --all-features -- -D warnings
• cargo build
• ./target/debug/kagi auth check
• ./target/debug/kagi ask-page https://rust-lang.org/ "What is this page about?"
• ./target/debug/kagi ask-page https://rust-lang.org/ "What learning resources does this page mention?"
• invalid URL, invalid scheme, and empty-question failure paths
• ./scripts/demo-ask-page.sh

Notes

• there is no separate ask-page endpoint; the live browser flow reuses the normal Assistant prompt stream
• page context is activated by sending the prompt as URL + newline + question