feat(cowork): read cloud sessions and carry associated files

[NishantJoshi00]

Cloud-backed Claude Cowork sessions can now be pulled explicitly by session ID, Cowork URL, or a Cowork session's /chat/… alias. Access reuses the signed-in Claude Desktop account. Cloud sessions remain opt-in and are excluded from automatic local discovery.

Uploaded and generated files now retain their contents, names, and message associations through native, Common, and Simple export. This includes Cowork attachments, Claude Chat research reports, and create_file outputs. Local continuation writes the files into the destination session without overwriting earlier copies. Missing referenced files produce an explicit error.

Fixes #61.

Related to #62. This PR covers txcript's file ingestion, export, and continuation. Skillsync CLI sync and desktop/web file display require the companion application changes, so #62 should remain open until that complete path is shipped.

Validation:

• Workspace tests and strict Clippy with all features; featureless library Clippy; WASM build with search.
• Binary attachment fixtures cover native/Common/Simple round trips and continuation into Cowork, Claude Code, and Codex after removing the source files.
• Read-only live checks cover Cowork URLs and chat aliases. A research report and two Markdown files matched the source bytes through export and isolated Claude Code/Codex continuation. Private transcripts and credentials are not included in fixtures.
• All CI checks pass on the current head, including Windows, MSRV, package dry runs, and the CLI release build.

Cloud access is read-only. Path-based downloads retrieve current contents, not historical file versions. Downloads are capped at 64 MB per file and 128 MB per session. A live cloud session with a binary document was not available; binary download behavior is covered by HTTP contract fixtures.