docs: add conda fallback install command

docs/installation.md

@@ -33,6 +33,18 @@ If the environment already exists:
 conda activate skills-vault
 ```
 
+If `conda env create -f environment.yml` fails in Codex or another non-interactive Windows shell because of progress output, solver plugin, or root environment directory issues, create the environment explicitly under the user env directory:
+
+```powershell
+C:\Users\wangq\miniconda3\Scripts\conda.exe create -p C:\Users\wangq\.conda\envs\skills-vault python=3.11 pip -y --json --solver classic
+```
+
+Then use:
+
+```powershell
+conda activate C:\Users\wangq\.conda\envs\skills-vault
+```
+
 Most lightweight Skills should use this shared environment. Create a dedicated environment only when a Skill has conflicting or heavy dependencies.
 
 ## Install One Skill