Add generic agent support with customizable command directories (#1639)

- Add --ai generic option for unsupported AI agents (bring your own agent)
- Require --ai-commands-dir to specify where agent reads commands from
- Generate Markdown commands with $ARGUMENTS format (compatible with most agents)
- Rebuild CHANGELOG from GitHub releases (last 10 releases)
- Align version to 0.1.3

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/test_ai_skills.py

@@ -380,7 +380,7 @@ class TestInstallAiSkills:
         # .toml commands should be untouched
         assert (cmds_dir / "speckit.specify.toml").exists()
 
-    @pytest.mark.parametrize("agent_key", list(AGENT_CONFIG.keys()))
+    @pytest.mark.parametrize("agent_key", [k for k in AGENT_CONFIG.keys() if k != "generic"])
     def test_skills_install_for_all_agents(self, temp_dir, agent_key):
         """install_ai_skills should produce skills for every configured agent."""
         proj = temp_dir / f"proj-{agent_key}"