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>

gsd-core/workflows/help/modes/full.md

@@ -86,7 +86,7 @@ Create detailed execution plan for a specific phase.
 
 - `--skip-research` — bypass the research subagent
 - `--research-phase <N>` — research-only mode. Spawns the research agent for phase `<N>`, writes `RESEARCH.md`, then exits before the planner runs. Useful for cross-phase research, doc review before committing to a planning approach, and correction-without-replanning loops. Replaces the deleted `gsd-research-phase` standalone command (#3042).
-  - Modifiers: `--research` forces refresh (re-spawn researcher, no prompt). `--view` prints existing `RESEARCH.md` to stdout without spawning. With neither, prompts `update / view / skip` if `RESEARCH.md` already exists.
+  - Modifiers: `--research` forces refresh (re-spawn researcher). `--view` prints existing `RESEARCH.md` to stdout without spawning. With neither, auto-uses an existing `RESEARCH.md` (one-line notice, then clean exit).
 - `--gaps` — focus only on closing gaps from a prior plan-check
 - `--skip-verify` — skip the post-plan verifier loop
 - `--ingest <path-or-glob>` — pre-ingest external ADRs/PRDs/SPECs before planning (see *PRD Express Path* below)
@@ -100,7 +100,7 @@ Create detailed execution plan for a specific phase.
 - Multiple plans per phase supported (XX-01, XX-02, etc.)
 
 Usage: `/gsd:plan-phase 1`
-Usage: `/gsd:plan-phase --research-phase 2` — research only on phase 2 (prompts if `RESEARCH.md` exists)
+Usage: `/gsd:plan-phase --research-phase 2` — research only on phase 2 (auto-uses existing `RESEARCH.md`, no prompt)
 Usage: `/gsd:plan-phase --research-phase 2 --view` — print existing `RESEARCH.md`, no spawn
 Usage: `/gsd:plan-phase --research-phase 2 --research` — force-refresh, no prompt
 Result: Creates `.planning/phases/01-foundation/01-01-PLAN.md`