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/load-contracts.sh

@@ -33,5 +33,23 @@ for f in $FILES; do
     printf '\n' >> "$TEMP"
 done
 
+# Sanctioned mission injection point (M4): when the task runner provides a
+# mission snapshot, its objective and directives are appended AFTER the
+# immutable contracts. Runtime data; never part of the contract fixtures.
+if [ -n "${MOSAIC_MISSION_FILE:-}" ]; then
+    if [ ! -r "$MOSAIC_MISSION_FILE" ]; then
+        echo "load-contracts: MOSAIC_MISSION_FILE set but not readable: $MOSAIC_MISSION_FILE" >&2
+        rm -f "$TEMP"
+        exit 1
+    fi
+    printf '===== MISSION (runtime) =====\n' >> "$TEMP"
+    node -e '
+const m = JSON.parse(require("fs").readFileSync(process.env.MOSAIC_MISSION_FILE, "utf8"));
+process.stdout.write("Objective: " + m.objective + "\n");
+for (const d of m.directives ?? []) process.stdout.write("- " + d + "\n");
+' >> "$TEMP"
+    printf '\n' >> "$TEMP"
+fi
+
 mv "$TEMP" "$OUT"
 echo "load-contracts: wrote $OUT from $CONTRACT_DIR" >&2