feat: add OMP (Oh My Pi) provider compatibility

Support OMP as a host alongside pi:
- Convert OMP's array-format system prompts to string via systemPromptToText
- Guard against OMP passing the literal env-var name as the API key value
- Add ~/.omp/agent/auth.json to default auth path lookup
- Fix --list-models output to check both stdout and stderr
- Add OMP compatibility smoke test with isolated temp HOME and mock server
- Deduplicate usage section in README, add OMP install and usage docs

tests/test-pi-local.mjs

@@ -136,6 +136,7 @@ function hasLivePiAuth() {
   return (
     !!process.env.COMMANDCODE_API_KEY ||
     existsSync(join(homedir(), ".commandcode", "auth.json")) ||
+    existsSync(join(homedir(), ".omp", "agent", "auth.json")) ||
     existsSync(join(homedir(), ".pi", "agent", "auth.json"))
   )
 }
@@ -294,9 +295,10 @@ try {
   modelListRequestCount = 0
   const list = await runPi(["--no-extensions", "-e", EXT_PATH, "--list-models"], 20_000)
   assert.equal(list.code, 0, list.stderr)
-  assert.match(list.stdout, /commandcode/)
-  assert.match(list.stdout, /deepseek\/deepseek-v4-flash/)
-  assert.match(list.stdout, /Qwen\/Qwen3\.7-Max/)
+  const listOutput = list.stdout || list.stderr
+  assert.match(listOutput, /commandcode/)
+  assert.match(listOutput, /deepseek\/deepseek-v4-flash/)
+  assert.match(listOutput, /Qwen\/Qwen3\.7-Max/)
   assert.equal(modelListRequestCount, 1)
 
   console.log("[pi-local] print mode through real extension and mock API")