BBC2-14 add bb pr diff

[b2l]

Summary

New bb pr diff [<id>] command. Defaults to the PR for the current branch. JSON mode wraps as {"diff": "..."}.

Implementation notes

• GET /pullrequests/{id}/diff returns a 302 to /repositories/{ws}/{slug}/diff/{spec} which serves text/plain. The typed openapi-fetch client is JSON-in/JSON-out; rather than fight it, this call uses raw fetch() with Accept: text/plain and redirect: follow. Same precedent as paginate.ts's raw-fetch cursor follower (noted in docs/bb-notes.md).
• Renderer interface gets a json: boolean flag so commands whose natural output is a raw blob (like a diff) can branch cleanly between "write bytes verbatim" and "emit JSON object." Tiny addition, used exactly once here.
• No query-param flags in v1 (per ticket). /diff/{spec} supports context, path, ignore_whitespace, binary, renames — easy to add later if someone asks.

Test plan

• bun src/index.ts pr diff <id> on a real PR prints the unified diff. Pipe to less — scrolling works.
• Pipe to delta (if you have it): bun src/index.ts pr diff <id> | delta — renders colorized.
• bun src/index.ts pr diff <id> --json via global --json flag: bun src/index.ts --json pr diff <id> emits {"diff": "..."}.
• No-id form: from a branch with an open PR, bun src/index.ts pr diff auto-detects.
• On a non-existent PR: clean error, exit 1 (not a stack trace).
• Tests: bun test (155 passing, 3 new).
• Lint: bun run lint.

Out of scope

• Filtering by path / context / whitespace flags.
• Structured hunks in JSON mode (users pipe to delta / similar for rendering).
• Color on TTY — we pass bytes verbatim; coloring is the user's pipeline.