feat(config): wire launcher scripts and compose to config.json (#2)

- common.sh: load_config() exports MOSAIC_DATA_ROOT/PROVIDER/MODEL; fails closed
- compose.yaml: dataRoot mount and provider/model are required env (:? errors)
- build/hello/verify load config before any mutation; no silent bootstrap
- reset.sh: target resolved from configured dataRoot; all safety checks kept

Verified: compose fails without launcher env; verify/reset fail on missing
config; config-driven hello+verify pass; symlink refusal with sandboxed
config (canary survived); config checksum unchanged across reset+rerun.

Closes #2

compose.yaml

@@ -6,17 +6,18 @@ services:
     image: mosaic-poc-agent:0.84.4
     user: "1000:1000"
     environment:
-      # Non-secret settings (see .env.example)
-      PI_PROVIDER: ${PI_PROVIDER:-zai}
-      PI_MODEL: ${PI_MODEL:-glm-5.3-flash}
+      # Resolved from config.json by scripts/common.sh (load_config).
+      # Required: compose fails fast when the launcher did not supply them.
+      PI_PROVIDER: ${MOSAIC_PROVIDER:?MOSAIC_PROVIDER must be set by scripts/load_config (run via scripts/*.sh)}
+      PI_MODEL: ${MOSAIC_MODEL:?MOSAIC_MODEL must be set by scripts/load_config (run via scripts/*.sh)}
       # Documented container auth alternative: provider API key via
       # runtime environment variable. Empty by default; when empty Pi
       # falls back to the read-only mounted auth.json credential file.
       ZAI_API_KEY: ${ZAI_API_KEY:-}
       ANTHROPIC_API_KEY: ${ANTHROPIC_API_KEY:-}
     volumes:
-      # Generated runtime state (host dir per brief)
-      - /home/jwoltje/.mosaic-dev:/var/lib/mosaic
+      # Configured runtime state root (from config.json dataRoot).
+      - ${MOSAIC_DATA_ROOT:?MOSAIC_DATA_ROOT must be set by scripts/load_config (run via scripts/*.sh)}:/var/lib/mosaic
       # Runtime credential only: pi auth file mounted READ-ONLY.
       # Never copied into the image.
       - ${PI_AUTH_FILE:-/home/jwoltje/.pi/agent/auth.json}:/home/node/.pi/agent/auth.json:ro