feat(config): non-destructive opencode.jsonc.proposed sidecar (closes #8 batch item)

Completes the pi-devbox v1.1.4 "merge new defaults into preserved config"
idea, adapted to opencode-devbox's env-generated, JSONC-with-comments config
where an in-place merge would be destructive.

generate-config.py keeps its "never touch an existing config" guarantee and
adds a side-channel: when a live config exists, render the config it WOULD
generate for the current env + image defaults and write it to a NON-loaded
opencode.jsonc.proposed — but only when it differs from the live config;
remove it once they match. opencode never loads .proposed files, so it is a
pure manual-merge reference (e.g. surfacing a default MCP server added in a
newer image). An unparseable live config surfaces the proposal rather than
guessing equivalence. A one-line hint is logged on write.

- render_config(): shared renderer so first-gen and proposed paths can't drift
- _loads_jsonc(): string-aware //-comment strip (same approach as smoke-test;
  preserves https:// inside strings), raises on invalid JSON
- write_proposed(): write-on-diff + stale removal + live untouched
- smoke-test.sh: asserts write-on-diff, removal-on-match, live not clobbered
- entrypoint-user.sh + module docstring: document the sidecar
- CHANGELOG: moved from "Deferred" to "Added"

Caveat (documented in the file header): the proposal reflects env + image
defaults, so a diff may include the user's own past edits, not only new
image defaults.

scripts/smoke-test.sh

@@ -306,6 +306,37 @@ if docker run --rm \
 else
   fail "$label: existing config was modified!"
 fi
+
+# Proposed-config side-channel: when a config already exists, a NEWER default
+# config is surfaced as a NON-loaded opencode.jsonc.proposed (write-on-diff,
+# removed once the live config matches). The live config is never touched.
+label="generate-config writes .proposed only when config differs"
+if docker run --rm \
+    -e OPENCODE_PROVIDER=anthropic \
+    -e HOME=/tmp/home \
+    --entrypoint="" \
+    "$IMAGE" sh -c '
+      set -e
+      d=/tmp/home/.config/opencode
+      mkdir -p "$d"
+      gc=/usr/local/lib/opencode-devbox/generate-config.py
+      # (a) differing existing config → proposed written, live NOT clobbered
+      printf "{\n  \"model\": \"old/model\"\n}\n" > "$d/opencode.jsonc"
+      python3 "$gc" 2>/dev/null
+      test -f "$d/opencode.jsonc.proposed"
+      grep -q "old/model" "$d/opencode.jsonc"
+      # (b) live matches defaults + stale proposed present → proposed removed
+      rm -f "$d/opencode.jsonc" "$d/opencode.jsonc.proposed"
+      python3 "$gc" 2>/dev/null
+      cp "$d/opencode.jsonc" "$d/opencode.jsonc.proposed"
+      python3 "$gc" 2>/dev/null
+      test ! -f "$d/opencode.jsonc.proposed"
+      echo ok
+    ' 2>/dev/null | grep -q ok; then
+  pass "$label"
+else
+  fail "$label: proposed-config behaviour incorrect"
+fi
 rm -rf "$tmp"
 
 echo