fix(#2697): replace retired /gsd: prefix with /gsd- in all user-facing text (#2699)

All workflow, command, reference, template, and tool-output files that
surfaced /gsd:<cmd> as a user-typed slash command have been updated to
use /gsd-<cmd>, matching the Claude Code skill directory name.

Closes #2697

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

get-shit-done/workflows/validate-phase.md

@@ -28,7 +28,7 @@ AUDITOR_MODEL=$(gsd-sdk query resolve-model gsd-nyquist-auditor --raw)
 NYQUIST_CFG=$(gsd-sdk query config-get workflow.nyquist_validation --raw)
 ```
 
-If `NYQUIST_CFG` is `false`: exit with "Nyquist validation is disabled. Enable via /gsd:settings."
+If `NYQUIST_CFG` is `false`: exit with "Nyquist validation is disabled. Enable via /gsd-settings."
 
 Display banner: `GSD > VALIDATE PHASE {N}: {name}`
 
@@ -41,7 +41,7 @@ SUMMARY_FILES=$(ls "${PHASE_DIR}"/*-SUMMARY.md 2>/dev/null)
 
 - **State A** (`VALIDATION_FILE` non-empty): Audit existing
 - **State B** (`VALIDATION_FILE` empty, `SUMMARY_FILES` non-empty): Reconstruct from artifacts
-- **State C** (`SUMMARY_FILES` empty): Exit — "Phase {N} not executed. Run /gsd:execute-phase {N} ${GSD_WS} first."
+- **State C** (`SUMMARY_FILES` empty): Exit — "Phase {N} not executed. Run /gsd-execute-phase {N} ${GSD_WS} first."
 
 ## 2. Discovery
 
@@ -146,14 +146,14 @@ gsd-sdk query commit "docs(phase-${PHASE}): add/update validation strategy"
 ```
 GSD > PHASE {N} IS NYQUIST-COMPLIANT
 All requirements have automated verification.
-▶ Next: /gsd:audit-milestone ${GSD_WS}
+▶ Next: /gsd-audit-milestone ${GSD_WS}
 ```
 
 **Partial:**
 ```
 GSD > PHASE {N} VALIDATED (PARTIAL)
 {M} automated, {K} manual-only.
-▶ Retry: /gsd:validate-phase {N} ${GSD_WS}
+▶ Retry: /gsd-validate-phase {N} ${GSD_WS}
 ```
 
 Display `/clear` reminder.