address review: qualify token portability for skills-mode extensions

Copilot correctly noted the __SPECKIT_COMMAND_<NAME>__ token is not yet
resolved for extension-generated skills: _register_extension_skills()
calls resolve_skill_placeholders() and post_process_skill_content() but
never resolve_command_refs(), so the token reaches Codex/ZCode/Kimi
verbatim in skills mode. Token resolution only runs in the command-file
rendering path (CommandRegistrar). Add an explicit limitation note so the
guidance no longer implies universal portability.

Author: Quratulain-bilal

extensions/EXTENSION-DEVELOPMENT-GUIDE.md

@@ -292,6 +292,16 @@ Once the assessment exists, the next step is `__SPECKIT_COMMAND_BUG_FIX__ slug=<
 
 This renders as `/speckit.bug.fix slug=<slug>` for a slash-based agent, `/speckit-bug-fix slug=<slug>` for a skills-based agent, and so on — the author writes it once and it stays portable. The first-party `bug` and `git` extensions use this token exclusively; see `extensions/bug/commands/` for working examples.
 
+> **Current limitation — skills mode.** Token resolution runs in the
+> command-rendering path (`CommandRegistrar`), so it applies when an extension
+> installs *command files*. It does **not** yet run when an extension is
+> registered as *skills* for a skills-based agent: `_register_extension_skills`
+> resolves placeholders and post-processes content but never calls
+> `resolve_command_refs`, so a `__SPECKIT_COMMAND_<NAME>__` token reaches
+> agents such as Codex, ZCode, and Kimi verbatim in that mode. Until that
+> rendering step lands, prefer the token for command-file extensions and avoid
+> relying on it inside skill bodies destined for skills-based agents.
+
 ### Script Path Rewriting
 
 Extension commands use relative paths that get rewritten during registration: