fix(#2390): warn on goal-shaped phase.add titles, correct auto-detect docs (#2425)

* fix(#2390): phase.add title warning + auto-detect doc fix

phase.add now returns a `warning` field when a description reads as
goal-shaped (>80 chars and/or multi-sentence) rather than title-shaped,
instead of silently writing the whole paragraph verbatim as the
`### Phase N:` header. The CLI still creates the phase as-is (the
strict two-layer slash-vs-CLI interface is unchanged); the warning
just surfaces the gap.

Also clarifies six doc sites (command argument hints, workflow
detection steps, and how-to/reference docs) that described the
phase-number argument as "auto-detecting" the next unplanned phase --
that detection is an orchestrating-workflow/LLM step reading
ROADMAP.md (concretely: `query roadmap.analyze`'s `next_phase`
field), not a `gsd-tools.cjs` CLI feature.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(#2390): regenerate fixtures + lint gate-prep

* fix(#2390): repair failing tests after gate verification

* chore(#2390): add changeset (#2425)

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.changeset/2390-phase-add-autodetect.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 2425
+---
+**`phase.add` no longer silently mistakes a goal-shaped description for a phase title** — a long or multi-sentence description used to land verbatim in the `### Phase N:` header with no signal anything was off; `phase.add` now returns a `warning` field when the description looks goal-shaped, and the phase-number auto-detect docs now correctly point callers at the orchestrating workflow instead of implying `gsd-tools.cjs` resolves it itself. (#2390)