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>

commands/gsd/plan-phase.md

@@ -22,8 +22,8 @@ Create executable phase prompts (PLAN.md files) for a roadmap phase with integra
 **Research-only mode (`--research-phase <N>`):** Spawn `gsd-phase-researcher` for phase `N`, write `RESEARCH.md`, then exit before the planner runs. Useful for cross-phase research, doc review before committing to a planning approach, and correction-without-replanning loops where iterating on research alone is dramatically cheaper than re-spawning the planner. Replaces the deleted research-phase command (#3042).
 
 **Research-only modifiers:**
-- **No flag** — when `RESEARCH.md` already exists, prompt the user to choose `update / view / skip`.
-- **`--research`** — force-refresh: re-spawn the researcher unconditionally, no prompt. Skips the existing-RESEARCH.md menu.
+- **No flag** — when `RESEARCH.md` already exists, auto-uses it: emits a one-line notice and exits cleanly, no prompt.
+- **`--research`** — force-refresh: re-spawn the researcher unconditionally, no prompt. Bypasses the existing-RESEARCH.md auto-use path.
 - **`--view`** — view-only: print existing `RESEARCH.md` to stdout. Does not spawn the researcher. Cheapest mode for the correction-without-replanning loop. If no `RESEARCH.md` exists yet, errors with a hint to drop `--view`.
 
 **Orchestrator role:** Parse arguments, validate phase, research domain (unless skipped), spawn gsd-planner, verify with gsd-plan-checker, iterate until pass or max iterations, present results.