feat(config): flag stale default-branch cache in wt config state (#3471)

[worktrunk-bot]

Closes #3471.

What

wt config state's DEFAULT BRANCH section now calls out when the cached worktrunk.default-branch has drifted from git's local <remote>/HEAD — the scenario from #3471, where a GitHub default-branch rename plus git remote set-head origin -a leaves an existing clone silently branching wt switch --create off the stale base.

DEFAULT BRANCH
  old-default
▲ Cached branch differs from origin/HEAD (main)
↳ To adopt it, run wt config state default-branch set main; to re-detect, run wt config state default-branch clear

The check is local-only (reads the <remote>/HEAD symref — no git ls-remote, no network) and fires only when both the cache and <remote>/HEAD resolve and differ. No drift → just the value, as before.

Why on inspection, not per-command

As noted in the triage comment, worktrunk.default-branch doubles as the storage for an explicit user override (wt config state default-branch set). A per-command warning would fire on every command for anyone who intentionally pinned a non-origin/HEAD default. Surfacing it only when the user inspects state (matching the warning-placement guidance for "issues that persist until the user fixes them") sidesteps that entirely, which is what your wt config show / or similar suggestion pointed at.

Placement note: I put this in wt config state rather than wt config show because config state is where the default-branch cache is actually displayed (the DEFAULT BRANCH section), and the existing StaleDefaultBranch error already routes users to wt config state default-branch set. config show has no default-branch section today. Happy to move or mirror it into config show if you'd prefer it there.

Changes

• Repository::remote_head() — new local-only accessor returning the primary remote's HEAD as (remote, branch); never queries the remote or infers, never persists.
• wt config state (table): drift warning + set/clear hints in the DEFAULT BRANCH section.
• wt config state get --format=json: new remote_head_branch field so scripts can detect drift too.
• Docs: a note in the default-branch detection section explaining the cache isn't re-validated and how the drift call-out reconciles it.

Tests

• test_state_show_default_branch_drift_warns — cache stale vs origin/HEAD → warning renders.
• test_state_show_default_branch_no_drift_when_matching — cache matches → no warning.
• test_state_get_json_reports_remote_head_branch — JSON carries both values.

Verified end-to-end against a scratch repo reproducing the exact #3471 steps.

[worktrunk-bot]

The drift check is clean — remote_head() reuses primary_remote()/local_default_branch(), stays local-only (no ls-remote), and correctly returns None (no false warning) when <remote>/HEAD is unset. One convention nit inline, plus one design point for a human call.

Override vs. drift framing (for @max-sixty to weigh): since worktrunk.default-branch doubles as an intentional override, a user who deliberately ran default-branch set develop while origin/HEAD → main now sees, on every wt config state, "Cached branch differs from origin/HEAD (main)" plus a hint to set main — i.e. advice to clobber their deliberate choice. The code comment acknowledges the doubling; the open question is whether the wording ("differs" / "To adopt it") should soften when the value looks like an intentional pin. Not blocking — just flagging the UX for the maintainer's call.

[worktrunk-bot]

The writing-user-outputs skill flags this pattern: hints render as a separate message from the warning above, so "it" (the remote's HEAD branch named on the warning line) has no local referent — Avoid pronouns with cross-message referents … Don't use pronouns like "it" that refer to something mentioned in the error message. Naming {remote}/HEAD directly makes the hint self-contained (remote is already in scope):

Suggested change

	"To adopt it, run <underline>wt config state default-branch set {remote_head}</>; to re-detect, run <underline>wt config state default-branch clear</>"
	"To adopt <bold>{remote}/HEAD</>, run <underline>wt config state default-branch set {remote_head}</>; to re-detect, run <underline>wt config state default-branch clear</>"