fix: watch source mode config files for multi-mode hot reload

[dolepee]

Overview

Extends the multi-mode hot-reload system to watch source configuration files (and their referenced sub-configs), not just the generated runtime config snapshot.

Previously, ModeCortexRuntime only monitored the runtime config file for changes. If a user edited the original source config or any mode file it references, changes were silently ignored until a full restart. This patch closes that gap.

Relates to #984

Type of change

• Bug fix
• New feature

Changes

• Source config resolution (_get_source_config_path): resolves the original mode config file from mode_config_name, handling absolute paths, relative paths, and the default config/ directory.
• Config reference extraction (_extract_config_references): parses a JSON5 config and recursively discovers any .json/.json5 file paths referenced as string values (skipping URLs).
• Multi-file watch set (_get_watched_config_paths): builds the complete set of files to monitor — runtime config + source config + all referenced configs.
• Mtime tracking (_refresh_watched_config_mtimes): replaces the single last_modified check with a dict of per-file mtimes so any watched file change triggers a reload.
• Smart reload path (_reload_config(reload_from_runtime=...)): when only the runtime snapshot changed, reloads from the snapshot; when source files changed, reloads from the original source and regenerates the runtime snapshot to keep them in sync.

Checklist

• Code complies with style guidelines
• Self-review completed
• Tests added/updated (if applicable)
• Local testing completed

Impact

This is scoped entirely to multi_mode/cortex.py and its tests. It does not touch single-mode, does not add new dependencies (uses the existing json5 import), and is fully backward-compatible — the default behaviour when only the runtime config changes is identical to before.

Additional Information

This is complementary to the single-mode selective field reload work discussed in #984. That bounty focuses on partial field-level reloading in single_mode/cortex.py; this PR addresses a different gap — ensuring multi-mode source config files and their references are properly watched alongside the runtime snapshot.