fix(#2543): replace legacy /gsd-<cmd> syntax with /gsd:<cmd> across all source files (#2595)

Commands are now installed as commands/gsd/<name>.md and invoked as
/gsd:<name> in Claude Code. The old hyphen form /gsd-<name> was still
hardcoded in hundreds of places across workflows, references, templates,
lib modules, and command files — causing "Unknown command" errors
whenever GSD suggested a command to the user.

Replace all /gsd-<cmd> occurrences where <cmd> is a known command name
(derived at runtime from commands/gsd/*.md) using a targeted Node.js
script. Agent names, tool names (gsd-sdk, gsd-tools), directory names,
and path fragments are not touched.

Adds regression test tests/bug-2543-gsd-slash-namespace.test.cjs that
enforces zero legacy occurrences going forward. Removes inverted
tests/stale-colon-refs.test.cjs (bug #1748) which enforced the now-obsolete
hyphen form; the new bug-2543 test supersedes it. Updates 5 assertion
tests that hardcoded the old hyphen form to accept the new colon form.

Closes #2543

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

tests/execute-phase-wave.test.cjs

@@ -101,7 +101,7 @@ describe('execute-phase docs: user-facing wave flag', () => {
       'help.md should describe wave-specific execution'
     );
     assert.ok(
-      content.includes('Usage: `/gsd-execute-phase 5 --wave 2`'),
+      content.includes('Usage: `/gsd:execute-phase 5 --wave 2`') || content.includes('Usage: `/gsd-execute-phase 5 --wave 2`'),
       'help.md should include wave-filter usage'
     );
   });