docs: lock device to one linear branch via Publish-is-sync principle

The user's mental model of Publish is a Google Doc backed by git, not a
curated commit log. Branches fall out of scope under that framing; the
device tracks one stream of work on whichever branch the remote was
cloned on, and never switches. Drops :Gbranch from v0.7 to match.

CONTEXT.md

@@ -86,6 +86,12 @@ into user-facing language).
   reconcile remote state in the background. If a previous **Publish** ended
   mid-flight and left a local commit unpushed, the next user-initiated
   **Publish** picks it up; until then, the device is silent about it.
+- **Publish is sync, not history.** The user's mental model is a Google Doc
+  that happens to be backed by git: the point is "I want to read this on my
+  phone later," not "I want a curated commit log." Commits are a transport
+  detail the device authors itself. Branches are out of scope for the same
+  reason — the device tracks one linear stream of work on whichever branch
+  the remote was cloned on, and never switches.
 - **No state the user didn't ask for.** No banners about pending work, no
   prompts about divergence, no "did you mean to publish" warnings. The status
   line reflects the _current_ action's outcome, nothing else.

docs/roadmap.md

@@ -71,7 +71,6 @@ Out of scope: Vim, palette, multiple files, branches, conflict handling.
 
 - [ ] `/` forward search, `n N`
 - [ ] `:Gpull` (fetch + fast-forward only; refuse on conflict and surface it)
-- [ ] `:Gbranch` to switch branches; refuse with dirty tree
 
 ## v0.8 — Power: battery + sleep — [ ]