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`

gsd-core/workflows/plan-phase.md

@@ -417,15 +417,15 @@ Pass `ai_spec_path` and `framework_line` to planner in step 7 so it can referenc
 
 **Skip if:** `--gaps` flag or `--skip-research` flag or `--reviews` flag.
 
-### 5.0. Research-Only Modifiers (`--view`, `--research`, prompt)
+### 5.0. Research-Only Modifiers (`--view`, `--research`)
 
 **Skip if:** `RESEARCH_ONLY` is `false`.
 
 Three branches in research-only mode (`--research-phase <N>`):
 
-1. **`--view`** (or user picks "View" in the prompt below): print `RESEARCH.md` to stdout, no spawn, exit. If `RESEARCH.md` is missing, error with: `--view requires an existing RESEARCH.md; drop --view to spawn the researcher.`
+1. **`--view`**: print `RESEARCH.md` to stdout, no spawn, exit. If `RESEARCH.md` is missing, error with: `--view requires an existing RESEARCH.md; drop --view to spawn the researcher.`
 2. **`--research`** (force-refresh): re-spawn researcher unconditionally — fall through to "Spawn gsd-phase-researcher" below.
-3. **Neither flag AND `has_research=true`:** emit `RESEARCH.md already exists for Phase ${PHASE}.` and prompt the user with three choices: `1. Update — re-spawn researcher and refresh RESEARCH.md`, `2. View — print existing RESEARCH.md and exit (no spawn)`, `3. Skip — exit without spawning or printing`. Map "Update" → fall through to spawn, "View" → set `VIEW_ONLY=true` and emit RESEARCH.md as in (1), "Skip" → exit cleanly. Mirrors the deleted `/gsd-research-phase` standalone's existing-artifact menu (#3042 parity).
+3. **Neither flag AND `has_research=true`:** auto-use the existing research and exit cleanly — do not prompt, do not re-spawn. Emit `RESEARCH.md already exists for Phase ${PHASE}, using it. To force-refresh, re-invoke with --research; to print, re-invoke with --view. Path: ${research_path}` then exit. The explicit-flag escape hatches cover any deviation; this matches §5.1's promptless auto-use of existing research, removing the §5.0/§5.1 inconsistency (#159).
 
 ```bash
 if [[ "$VIEW_ONLY" == "true" ]]; then