feat: add full-page screenshots to extension and CLI

[iuyo5678]

Adds long screenshots to Quick actions and bsk screenshot --full-page. The popup supports automatic document scrolling, manual scrolling, visible-area capture, pause/resume, early finish, cancellation, tiled preview and PNG download. Quick Actions works independently of the CLI. Finished popup state is cleared when the user changes pages; existing previews remain usable.

Agents can use bsk screenshot --session <id> --full-page --out page.png, with an optional --timeout (default: two minutes for capture and encoding). This reuses the same scrolling, disk tiles and PNG encoder. The extension excludes the Agent overlay, restores page position/styles, and serves the PNG in session-scoped 256 KiB chunks. The CLI atomically replaces the output after receiving all bytes and releases the export after success or failure. Ctrl-C and the page Escape control cancel the operation. Full-page scrolling obeys session ownership and user-interrupt gates; viewport and ref screenshots retain their existing route and output shape.

Captures use 512-row OPFS tiles and streaming PNG encoding, avoiding a full-page canvas or whole-image base64 transport. Preview only loads nearby tiles. There is no fixed 32K-height or megapixel cutoff; storage capacity, PNG bounds and viewer capabilities still apply. Quick Actions can retain durable partial results, while Agent failures never report a partial image as a completed capture. Expired and abandoned Agent exports are cleaned separately from user previews.

Automatic capture reconnects its content script after extension reload and observes top-level navigation rather than subframe loading. Restricted browser/store pages support user-invoked manual or visible-area capture; Agent automatic capture requires a scriptable HTTP(S) document. Nested scroll containers and virtualized lists remain outside automatic full-page support. Manual alignment rejects ambiguous overlaps instead of guessing seams.

Validation:

• 1,322 extension tests and 649 Rust tests passed; TypeScript, production builds/ZIP, formatting and CSS lint passed.
• Five isolated CLI → daemon → Chrome scenarios passed: exact normal/Retina PNG scanlines and fixed footer, multi-chunk transfer, a 34,034-pixel page, timeout, Ctrl-C, page Escape, output errors, page restoration and temporary-file cleanup.
• Ten real extension UI scenarios passed, including page-state reset, retained preview export, extension reload recovery, automatic/manual capture, pause/resume/early finish, cancellation, native download, Chrome internal pages and the Web Store.
• Renderer PNG pixel regression passed. Capture, streaming PNG and alignment unit tests retain coverage of fractional offsets, large outputs, lazy content, cancellation and storage errors.

CLI help, protocol schemas, English/Chinese READMEs, the bundled Agent skill and docs/long-screenshot.md are updated. Use matching CLI and extension builds and restart an already-running daemon after upgrading the CLI.