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/pi/adapter.sh

@@ -0,0 +1,33 @@
+#!/bin/sh
+# Pi adapter: implements the Mosaic adapter contract for the pinned
+# @earendil-works/pi-coding-agent CLI.
+#
+# Contract: see /opt/mosaic/adapters/README.md. stdout = response only.
+set -eu
+
+[ -n "${MOSAIC_SYSTEM_PROMPT_FILE:-}" ] || { echo "pi adapter: MOSAIC_SYSTEM_PROMPT_FILE is required" >&2; exit 2; }
+[ -n "${MOSAIC_REQUEST:-}" ] || { echo "pi adapter: MOSAIC_REQUEST is required" >&2; exit 2; }
+[ -r "$MOSAIC_SYSTEM_PROMPT_FILE" ] || { echo "pi adapter: system prompt not readable: $MOSAIC_SYSTEM_PROMPT_FILE" >&2; exit 2; }
+
+: "${PI_PROVIDER:?pi adapter: PI_PROVIDER is required}"
+: "${PI_MODEL:?pi adapter: PI_MODEL is required}"
+
+# All flags documented in the pi package README (CLI Reference):
+#   -p/--print     noninteractive: print the response and exit
+#   --system-prompt  replace the default prompt with the generated one
+#   --no-*         no ambient context/skills/extensions/templates/themes
+#   --no-session   ephemeral; --no-tools  this runtime needs no tools
+#   --offline      no startup network operations (update checks/telemetry)
+exec pi \
+    --offline \
+    --no-session \
+    --no-extensions \
+    --no-skills \
+    --no-prompt-templates \
+    --no-themes \
+    --no-context-files \
+    --no-tools \
+    --provider "$PI_PROVIDER" \
+    --model "$PI_MODEL" \
+    --system-prompt "$(cat "$MOSAIC_SYSTEM_PROMPT_FILE")" \
+    -p "$MOSAIC_REQUEST"