docs: describe channel-backed task experiment

Signed-off-by: John Tennant <jtennant@squareup.com>

docs/experiments/channel-backed-tasks.md

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+# Channel-backed task experiment
+
+This is a living record of experiments based on [Buzz Channels — One Conversation, Many Views][channel-vision]. We are testing whether one Buzz channel can remain the shared home for a conversation as task, branch, and pull-request views appear.
+
+## Currently testing
+
+```text
+Conversation → local task + new channel → implementation branch → pull request
+```
+
+- The task's mutable data lives in the channel canvas.
+- No native Buzz task is created.
+- The task has zero or one implementation branch.
+- The branch, pull request, CI, and review use the task's channel.
+
+## Agent decision flow
+
+### When asked to create a task
+
+1. Is the current channel scoped only to this feature?
+   - **Yes:** Add the task details to the current channel canvas.
+   - **No:** Create a channel and add the task details to its canvas.
+
+### When asked to start work on a feature
+
+1. Is the current channel scoped only to this feature?
+   - **No:** Create a channel for the feature and continue below using the new channel.
+   - **Yes:** Continue below using the current channel.
+2. Does the current channel have task details?
+   - **No:** Add them to the canvas.
+3. Does the current channel have a branch?
+   - **No:** Create one and add its details to the canvas.
+4. Start work.
+
+## Future experiments
+
+- Start with a bare channel, then add a task or branch view.
+- Start with a branch channel, then add a task view.
+- Represent work requiring multiple implementation branches as a parent task with branch-backed subtasks.
+- Turn this decision flow into an API so agents declare their intent and Buzz creates or reuses the correct channel and bindings.
+
+We will update this document with what each experiment teaches us and which behavior requires native client or relay support.
+
+[channel-vision]: https://github.com/block/buzz/blob/2b1ccbab542c42f227f9714e13eb7e1377bcbe16/VISION_CHANNELS.md#L1