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.

entrypoint-user.sh

@@ -95,9 +95,11 @@ fi
 
 # ── Generate opencode config from env vars if no config mounted ──────
 # Delegated to a standalone Python script for clarity and testability.
-# The script is idempotent: it never overwrites an existing opencode.json
-# (bind-mounted from host, persisted in named volume, or previously
-# generated) and no-ops if OPENCODE_PROVIDER is unset.
+# The script never overwrites an existing opencode.json/.jsonc (bind-mounted
+# from host, persisted in named volume, or previously generated) and no-ops if
+# OPENCODE_PROVIDER is unset. When a config already exists it instead writes a
+# NON-loaded opencode.jsonc.proposed sidecar (only when newer image defaults
+# differ) for manual review/merge.
 python3 /usr/local/lib/opencode-devbox/generate-config.py
 
 # ── Skillset: deploy skills/instructions from mounted skillset repo ──