Fix aged stream rollover and file upload auth

[pzs19]

This fixes two SDK issues hit by long-running agent replies.

• Roll markdown streaming cards before they age out, then continue in a follow-up card.
• Pass a tenant access token to im.v1.file.create for generic file uploads.
• Add prepare so installing the package from a Git branch builds the exported dist files.

Tests:

• pnpm test -- --run src/outbound/__tests__/markdown-stream-rollover.test.ts src/outbound/__tests__/uploader.test.ts
• pnpm typecheck
• npm pack --dry-run

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