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>

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

@@ -13,7 +13,7 @@ Parse the command arguments:
 - First argument: integer phase number to insert after
 - Remaining arguments: phase description
 
-Example: `/gsd-insert-phase 72 Fix critical auth bug`
+Example: `/gsd:insert-phase 72 Fix critical auth bug`
 -> after = 72
 -> description = "Fix critical auth bug"
 
@@ -21,8 +21,8 @@ If arguments missing:
 
 ```
 ERROR: Both phase number and description required
-Usage: /gsd-insert-phase <after> <description>
-Example: /gsd-insert-phase 72 Fix critical auth bug
+Usage: /gsd:insert-phase <after> <description>
+Example: /gsd:insert-phase 72 Fix critical auth bug
 ```
 
 Exit.
@@ -99,7 +99,7 @@ Project state updated: .planning/STATE.md
 
 `/clear` then:
 
-`/gsd-plan-phase {decimal_phase}`
+`/gsd:plan-phase {decimal_phase}`
 
 ---
 
@@ -115,11 +115,11 @@ Project state updated: .planning/STATE.md
 
 <anti_patterns>
 
-- Don't use this for planned work at end of milestone (use /gsd-add-phase)
+- Don't use this for planned work at end of milestone (use /gsd:add-phase)
 - Don't insert before Phase 1 (decimal 0.1 makes no sense)
 - Don't renumber existing phases
 - Don't modify the target phase content
-- Don't create plans yet (that's /gsd-plan-phase)
+- Don't create plans yet (that's /gsd:plan-phase)
 - Don't commit changes (user decides when to commit)
 </anti_patterns>