feat(agent): add file uploads (chat attachments + workspace files)

[ThomasK33]

What

Adds file uploads to @coder/ai-sdk-agent, split by intent rather than by transport:

• Chat attachments — content for the model to read (PDF, image, CSV, …). A native AI SDK file part dropped into a message is uploaded to chat-file storage transparently:

  await agent.generate({ messages: [{ role: "user", content: [
    { type: "text", text: "Summarize this." },
    { type: "file", data: await readFile("report.pdf"), mediaType: "application/pdf", filename: "report.pdf" },
  ]}]});

  Or pre-upload once and reuse across turns (accepts Blob/File/stream/bytes):

  const f = await agent.attach({ content: await openAsBlob("report.pdf"), mediaType: "application/pdf" });
  f.toFilePart(); // references by id — no re-upload

  Validated against the 10 MiB cap and the media-type allowlist up front, with actionable errors.

• uploadToWorkspace() — material for the agent to operate on (zips, datasets, binaries — anything over the cap or off the allowlist). Writes bytes as-is via an injected WorkspaceFileStore adapter, keeping the agent core dependency-free. No unpacking — instruct the agent to unzip, or do it over your own connection.

How

• CoderChatClient.uploadChatFile() / getChatFile() wrap POST/GET /api/experimental/chats/files (raw body, not multipart), mirroring codersdk.
• FileContent = Blob | ReadableStream | Uint8Array | ArrayBuffer; Blob/File carry their own type/name/size, raw bytes are wrapped in a Blob for a clean body + Content-Length.
• Transparent file parts are uploaded inside CoderLanguageModel before the chat exists (the endpoint needs only the org id); a pre-uploaded id carried in providerOptions.coder.fileId is referenced without re-uploading.
• Removes the "file inputs not forwarded" limitation from the README and adds a Files section + examples/05-file-upload.ts.

Testing

• Unit (fake client): resolveFileContent normalization, transparent upload + providerOptions reuse, attach()/toFilePart(), uploadToWorkspace() with/without adapter. 34 unit tests pass.
• Live e2e against dev.coder.com (v2.34.3-devel): upload/download round-trip + model-reads-attachment. 5/5 e2e pass — the endpoint is wire-verified, not just source-derived.
• typecheck ✓ · oxlint ✓ · oxfmt ✓ · build ✓ (ESM + DTS).

Notes

• uploadToWorkspace() has no e2e (the suite is deliberately workspace-free, no provisioning); it's unit-tested with a fake adapter. A live test would need a running workspace + a real sandbox-session adapter.
• No client-side extract-text/chunk strategy yet (needs a PDF lib; intentionally kept out of core).

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