feat(assistant): add thread management commands

[Microck]

Summary

• expand kagi assistant with prompt-time controls and thread management
• add thread list, thread get, thread delete, and thread export
• refresh the Assistant docs and demo to match the new workflow

Implementation

• add authenticated Assistant thread transports for list, open, delete, and markdown export
• add prompt flags for --thread-id, --model, --lens, --web-access, --no-web-access, --personalized, and --no-personalized
• normalize full Session Link URLs on load so KAGI_SESSION_TOKEN='https://kagi.com/search?token=...' works in env and config
• add parser and contract coverage for Assistant prompt and thread flows
• improve invalid thread-id errors with a more actionable message

Docs and Demo

• rewrite the Assistant command reference for prompt mode and thread subcommands
• update README, auth matrix, coverage, output contract, API coverage, quickstart, and index docs
• update scripts/demo-assistant.sh
• record and ship fresh docs/demo-assets/assistant.gif and images/demos/assistant.gif

Verification

• cargo test
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test live_assistant_thread_roundtrip -- --ignored --nocapture
• live manual checks for prompt, thread list/get/export/delete, export json, prompt controls, invalid thread ids, and the demo script

Notes

• --mode is intentionally not exposed in this PR because no stable wire-level request field was confirmed
• Assistant lens override uses Assistant lens_id, not the search-command lens index
• this feature depends on Kagi's live Assistant web-product seam, so upstream contract drift remains the main long-term risk