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

src/run-agent.sh

@@ -1,38 +1,33 @@
 #!/bin/sh
-# One-shot Pi agent runner inside the container.
-# Loads the contract-generated system prompt, then sends exactly one
-# user request through Pi's documented noninteractive mode and prints
-# the model response on stdout.
+# One-shot agent dispatcher inside the container.
+#
+# 1. Loads the contract-generated system prompt (contracts + optional
+#    mission section from MOSAIC_MISSION_FILE).
+# 2. Dispatches to /opt/mosaic/adapters/<MOSAIC_ADAPTER>/adapter.sh per
+#    the contract in /opt/mosaic/adapters/README.md.
 set -eu
 
-: "${PI_PROVIDER:=zai}"
-: "${PI_MODEL:=glm-5.3-flash}"
-export PI_PROVIDER PI_MODEL
-
 REQUEST="${*:-Return your startup marker and nothing else.}"
 
+ADAPTER="${MOSAIC_ADAPTER:-pi}"
+case "$ADAPTER" in
+    # Allowlist mirrors scripts/mosaic-config.mjs; pattern check first so a
+    # crafted name cannot escape the adapters directory.
+    *[!A-Za-z0-9._-]*|'')
+        echo "run-agent: invalid adapter name: '$ADAPTER'" >&2
+        exit 2
+        ;;
+esac
+
+ADAPTER_SCRIPT="/opt/mosaic/adapters/$ADAPTER/adapter.sh"
+if [ ! -x "$ADAPTER_SCRIPT" ]; then
+    echo "run-agent: unknown or non-executable adapter: $ADAPTER" >&2
+    exit 2
+fi
+
 /opt/mosaic/src/load-contracts.sh /opt/mosaic/contracts /var/lib/mosaic/system-prompt.md
 
-# All flags are documented in the package README (CLI Reference):
-#   -p / --print        noninteractive: print the response and exit
-#   --system-prompt     replace the default system prompt with the
-#                       contract-generated prompt
-#   --no-* switches     prevent ambient context files, skills, extensions,
-#                       prompt templates, and themes from being appended
-#   --no-session        ephemeral: no persistent agent session
-#   --no-tools          the startup request needs no tool execution
-#   --offline           disable startup network operations (update checks,
-#                       package update checks, install/update telemetry)
-exec pi \
-    --offline \
-    --no-session \
-    --no-extensions \
-    --no-skills \
-    --no-prompt-templates \
-    --no-themes \
-    --no-context-files \
-    --no-tools \
-    --provider "$PI_PROVIDER" \
-    --model "$PI_MODEL" \
-    --system-prompt "$(cat /var/lib/mosaic/system-prompt.md)" \
-    -p "$REQUEST"
+export MOSAIC_SYSTEM_PROMPT_FILE="/var/lib/mosaic/system-prompt.md"
+export MOSAIC_REQUEST="$REQUEST"
+
+exec "$ADAPTER_SCRIPT"