Add durable draft resources with SDK and CLI parity

[brsbl]

Human comments

What was wrong

New-thread drafts existed only in browser storage, so SDK and CLI clients could not manage them while the app was closed.

What changed

• Add durable drafts with stable IDs, revision-checked edits, and retry-safe submission that preserves other drafts and newer edits.
• Expose create, list, show, update, delete, and submit through sdk.drafts and bb thread draft.
• Preserve saved draft attachments through upload cleanup; publish additive SDK types as version 0.4.102 and update CLI guides.

This is the approved clean-history replacement for #3501, based on 8d35c2776bc7fd33f41a64e4c6dd7ebd2f021887. It preserves the server-storage decision and accepted P1 fixes. The additive migration creates draft tables alongside existing data. The UI import and pane controls ship separately.

How you verified

• Remote CI is green at 720af27ce81921013712f1b1a3ad870eeaab220d, including migration, submission, attachment-retention, SDK, CLI, and package checks. Infrastructure download failures passed on rerun; no local CI-equivalent checks were run.
• Exact-head source server, Node 22.19.0: real SDK and CLI clients shared independent drafts with the app closed, rejected a stale revision, preserved model/destination through edits, returned the same scheduled thread on submission retry, and retained the second draft. Temporary runtime fixtures were removed.
• The sole review was completed on Add durable drafts to the SDK and CLI #3501; its accepted P1 fixes are retained. No additional review was performed.

BB-Thread-ID: thr_8j56at3ii2

AGENT GENERATED

[brsbl]

Closing as superseded at the user's request. The current implementation builds on the shipped Drafts plugin, threads, and queued messages: #3930 (SDK/CLI lifecycle filtering and save actions), #3933 (sidebar filters), #3938 (palette filters), and #3931 (independent New thread split composers). This PR is no longer a merge candidate. Its branch is retained.