feat(adapters): adapter contract, dispatch, pi + mock adapters (#16)

- adapters/README.md: the harness boundary contract (env in, response on
  stdout, diagnostics stderr, exit 0 success)
- adapters/pi: extracted current invocation unchanged
- adapters/mock: deterministic MOSAIC_MOCK_RESPONSE echo (test-only)
- run-agent.sh: name-validated dispatch to adapters/<name>/adapter.sh
- config: optional execution.adapter (pi|mock), default pi, configVersion
  stays 1 — existing configs remain valid; selection authority is the
  config file (load_config exports it)
- compose: MOSAIC_ADAPTER / MOSAIC_MOCK_RESPONSE passthrough; Containerfile
  installs adapters read-only; RELEASE -> 0.0.5

Verified: hello unchanged; mock verbatim via config; unknown adapter and
path-traversal names refused in-container; invalid adapter exits 2.

Closes #16

scripts/mosaic-config.mjs

@@ -30,6 +30,7 @@ import process from "node:process";
 const SUPPORTED_CONFIG_VERSION = 1;
 const SUPPORTED_ENVIRONMENTS = new Set(["development", "production"]);
 const SUPPORTED_BACKENDS = new Set(["docker"]);
+const SUPPORTED_ADAPTERS = new Set(["pi", "mock"]); // mock: test-only, see adapters/README.md
 const NAME_PATTERN = /^[A-Za-z0-9][A-Za-z0-9._:/-]{0,199}$/;
 
 function fail(exitCode, message) {
@@ -127,7 +128,7 @@ function validate(document, file) {
   if (!isPlainObject(document.execution)) {
     fail(2, '"execution" must be a JSON object');
   }
-  rejectUnknownKeys(document.execution, ["backend", "provider", "model"], '"execution"');
+  rejectUnknownKeys(document.execution, ["backend", "provider", "model", "adapter"], '"execution"');
   if (!SUPPORTED_BACKENDS.has(document.execution.backend)) {
     fail(2, `unsupported execution.backend: ${JSON.stringify(document.execution.backend)} (supported: ${[...SUPPORTED_BACKENDS].join(", ")})`);
   }
@@ -138,6 +139,13 @@ function validate(document, file) {
     }
   }
 
+  const adapter = document.execution.adapter === undefined || document.execution.adapter === null
+    ? "pi"
+    : document.execution.adapter;
+  if (typeof adapter !== "string" || !SUPPORTED_ADAPTERS.has(adapter)) {
+    fail(2, `unsupported execution.adapter: ${JSON.stringify(adapter)} (supported: ${[...SUPPORTED_ADAPTERS].join(", ")})`);
+  }
+
   return {
     configVersion: document.configVersion,
     environment: document.environment,
@@ -146,6 +154,7 @@ function validate(document, file) {
       backend: document.execution.backend,
       provider: document.execution.provider,
       model: document.execution.model,
+      adapter,
     },
   };
 }
@@ -222,6 +231,7 @@ switch (operation) {
         `MOSAIC_DATA_ROOT=${shellQuote(resolved.dataRoot)}`,
         `MOSAIC_PROVIDER=${shellQuote(resolved.execution.provider)}`,
         `MOSAIC_MODEL=${shellQuote(resolved.execution.model)}`,
+        `MOSAIC_ADAPTER=${shellQuote(resolved.execution.adapter)}`,
         "",
       ].join("\n"),
     );