BBC2-42 add --draft flag to bb pr create

[b2l]

Summary

• Adds --draft flag to bb pr create. When set, posts draft: true to Bitbucket; otherwise omits the field so the server-side default applies.
• Lands docs/bb-notes.md — verified Bitbucket Cloud API notes from the backlog grooming session. Referenced by upcoming tickets BBC2-40, 15, 17, 14, 18, 24, 25, 23.

Test plan

• `bun src/index.ts pr create --draft --title "test: draft PR" --body "..."` creates a draft PR — verify in the Bitbucket UI that it shows as a draft.
• `bun src/index.ts pr create --title "test: non-draft" --body "..."` creates a normal PR — verify not draft in UI.
• Backend tests pass: `bun test src/backend/pullrequests` (24 tests).
• Full test suite passes: `bun test` (131 tests).
• Lint clean: `bun run lint`.

Notes

• `--draft=false` (explicit) is treated identically to omitting the flag — we never send `draft: false` in the body. One test locks this in.
• The `docs/bb-notes.md` bundle is a one-file add; flagged in the commit message. If you'd rather split it into its own PR, happy to do so — it's a small change either way.