fix(#3207): create-and-switch on the first phase/milestone commit (#3363)

* test(#3207): invert fresh-create branching tests to expect create+switch

The four fresh-create branching tests (#3079) locked in create-without-
switch behavior that #3207 identifies as the regression: a fresh phase/
milestone branch is created but HEAD never moves onto it, so the first
phase/milestone-scoped commit lands on the base branch. Invert those
assertions to expect create+switch, and add two new tests:
- fresh-create is non-silent (logs the create+switch) (#3207 AC3)
- a second phase commit does not re-warn once HEAD is on the phase branch (#3207 AC5)

The existing-branch path (#2539) is deliberately byte-for-byte unchanged.

RED — fails on next; the fix follows in a separate fix: commit.

* fix(#3207): create-and-switch on the first phase/milestone commit

The #3079 fix (PR #3141) replaced git checkout -b with git branch
(create-only) unconditionally, including the case where the strategy
branch does not yet exist. That regressed #1278: the first phase- or
milestone-scoped commit no longer landed on the strategy branch — it
stayed on the base branch, and the strategy branch was left as an empty
marker pointing at the pre-phase tip. Every subsequent commit then
warned 'already exists; committing on the current branch instead of
switching', wording that misleads because the tool itself created the
branch moments before.

Re-separate the two cases #3079 collapsed:
- branch does NOT exist -> create AND switch (git checkout -b). The
  #3079 resurrection hazard cannot apply: the branch was just verified
  absent, so there is no merged-and-deleted ref to resurrect and no
  silent move onto an existing unrelated branch (#2539 AC2 is honored
  by the existing-branch arm, which is unchanged). The create is logged
  so the first phase-scoped commit is not silent (#3207 AC3).
- branch ALREADY exists -> unchanged: no switch, commit on current
  branch, non-silent warning (#2539/#3079).

Once the first commit switches HEAD onto the strategy branch, the
currentBranch !== branchName guard skips the block on subsequent
commits, so the misleading 'already exists' warning no longer recurs.

* chore(#3207): add changeset fragment

* chore(#3207): backfill changeset PR number (#3363)

---------

Co-authored-by: sim <sim@local>

.changeset/lazy-otter-purchase.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 3363
+---
+**`branching_strategy: "phase"`/`"milestone"` once again lands the first strategy-scoped commit on the strategy branch** — `gsd-tools query commit` now creates *and* switches to a brand-new phase/milestone branch (restoring the #1278 intent), instead of creating it without switching and leaving the commit on the base branch. The #3079 protection is preserved: an *already-existing* strategy branch is still never silently switched to (it warns and commits on the current branch). The first fresh create is now logged to stderr instead of being silent, and the misleading "already exists" warning no longer recurs on every subsequent commit once HEAD is on the strategy branch. (#3207)