feat(#159): auto-use existing RESEARCH.md in /gsd:plan-phase --research-phase (#718)

* feat(#159): auto-use existing RESEARCH.md in /gsd:plan-phase --research-phase

When RESEARCH.md already exists in research-only mode and neither --research
nor --view is passed, emit a one-line notice and exit cleanly instead of
prompting update/view/skip. This matches the promptless auto-use of standard
/gsd:plan-phase <N> (§5.1) and removes the §5.0/§5.1 inconsistency, making
AI-agent and CLI invocations non-interactive in the common case. The two
explicit-flag escape hatches (--research to refresh, --view to print) cover
any deviation.

Closes #159

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

* chore(#159): point changeset fragment at PR #718

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

* docs(#159): tighten research-phase reference register (Diataxis)

Make the 'no modifier' research-phase entries descriptive rather than
imperative and drop the trailing 'pass --research/--view' clauses, which
duplicated the adjacent --research/--view documentation. Reference docs
describe; the recovery flags are documented in their own entries. The
emitted runtime notice in the workflow keeps naming the flags (in-band
recovery), unchanged.

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

---------

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

docs/COMMANDS.md

@@ -162,7 +162,7 @@ Research, plan, and verify a phase.
 **Produces:** `{phase}-RESEARCH.md`, `{phase}-{N}-PLAN.md`, `{phase}-VALIDATION.md`; `{phase}/SKELETON.md` when Walking Skeleton mode fires
 
 **Research-only mode (`--research-phase <N>`):**
-- No modifier: prompts `update / view / skip` if RESEARCH.md already exists.
+- No modifier: when RESEARCH.md already exists, auto-uses it — emits a one-line notice and exits, no prompt.
 - With `--research`: force-refresh — re-spawn researcher unconditionally, no prompt.
 - With `--view`: print existing RESEARCH.md to stdout, no spawn. Errors if RESEARCH.md missing.
 
@@ -185,7 +185,7 @@ See [Package Legitimacy Gate in the User Guide](USER-GUIDE.md#package-legitimacy
 /gsd-plan-phase 1 --bounce                     # Plan + external bounce validation
 /gsd-plan-phase 2 --ingest docs/adr/0010.md   # ADR express path for context synthesis
 /gsd-plan-phase 2 --ingest 'docs/adr/00*.md' --ingest-format auto
-/gsd-plan-phase --research-phase 4             # Research only on phase 4 (prompts if RESEARCH.md exists)
+/gsd-plan-phase --research-phase 4             # Research only on phase 4 (auto-uses existing RESEARCH.md, no prompt)
 /gsd-plan-phase --research-phase 4 --view      # Print existing RESEARCH.md, no spawn
 /gsd-plan-phase --research-phase 4 --research  # Force-refresh research, no prompt
 /gsd-plan-phase 1 --mvp                        # Vertical-slice plan for phase 1