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

adapters/mock/adapter.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+# Mock adapter: deterministic response for seam tests. NEVER use where a
+# real model response is required.
+#
+# Contract: see /opt/mosaic/adapters/README.md.
+set -eu
+
+[ -n "${MOSAIC_SYSTEM_PROMPT_FILE:-}" ] || { echo "mock adapter: MOSAIC_SYSTEM_PROMPT_FILE is required" >&2; exit 2; }
+[ -n "${MOSAIC_REQUEST:-}" ] || { echo "mock adapter: MOSAIC_REQUEST is required" >&2; exit 2; }
+[ -r "$MOSAIC_SYSTEM_PROMPT_FILE" ] || { echo "mock adapter: system prompt not readable: $MOSAIC_SYSTEM_PROMPT_FILE" >&2; exit 2; }
+
+echo "mock adapter: responding verbatim from MOSAIC_MOCK_RESPONSE" >&2
+printf '%s\n' "${MOSAIC_MOCK_RESPONSE:-}"