fix(#160): resume partially-executed phases before current_phase routing (#406)

When a session dies mid-execution (hang, token exhaustion, API drop),
STATE.md's current_phase can be advanced past a phase that still has
PLAN.md files without matching SUMMARY.md files. Without this fix,
/gsd-next and /gsd-progress would route by current_phase and silently
skip the partially-executed phase, producing a data-loss-shape outcome.

Adds a Route 0 cross-phase incomplete-execution scan to both
next.md and progress.md. Before any current_phase-based routing,
the scan finds the lowest-numbered phase where plans outnumber
summaries and routes to /gsd-execute-phase <that-phase> to resume it.
Opt out with --no-resume to fall back to the prior-phase defer prompt;
--force bypasses all gates as before.

Rework (codex review):
- Route 0 now ordered AFTER Gates 1-3 (repo/state validity always run)
  but BEFORE the prior-phase completeness-scan defer prompt — eliminating
  the double-decision where the default path would both prompt the user
  (C/S/F) AND resume the phase anyway. Prior-phase defer prompt moved to
  a new prior_phase_completeness step; only reached via --no-resume.
- --force flow made coherent across all three steps: safety_gates jumps
  directly to determine_next_action, skipping Gates, Route 0, AND
  prior_phase_completeness. resume_incomplete_phase and prior_phase_completeness
  now correctly state --force never reaches them. success_criteria entry
  updated to reflect --force → determine_next_action (not prior_phase_completeness).
- Scan uses $GSD_SDK (canonical resolver form) throughout next.md, matching
  the file's existing convention. progress.md uses $ROADMAP already loaded
  by analyze_roadmap. Neither file uses bare gsd-sdk.
- Errors are surfaced rather than suppressed: removed 2>/dev/null on the
  main roadmap.analyze call; added explicit WARNING emission when the scan
  cannot run, so the invariant fails closed instead of failing open.
- Predicate aligned to plans-without-summaries (plans.length > summaries.length)
  in both files, consistent with determine_next_action Route 4.
- command references use canonical /gsd: namespace form

Fixes #160

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.changeset/160-route0-resume-incomplete-phase.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 160
+---
+`/gsd-next` and `/gsd-progress` no longer silently skip partially-executed phases when `current_phase` was advanced past unfinished work. New Route 0 invariant scans all phases for plans without summaries and routes to `/gsd-execute-phase <lowest-numbered>` before any current_phase-based routing decision. Opt out with `--no-resume` to use the existing prior-phase defer prompt instead; `--force` bypasses all gates.