- Auto-scan roles/*.json on startup and merge into built-in roles

[sufan721]

• Same title overrides built-in role; different title appends as new
• roles/ directory is optional — no impact when absent

Users can now add custom roles by simply dropping a .json file
into the roles/ directory and restarting. No config changes needed.

What does this PR do?

Previously, adding a custom role required editing roles.json directly,
which is bundled with the plugin and easily overwritten on updates.
This PR adds an optional roles/ directory: users can place individual
.json role files there, and they are auto-loaded and merged on startup.
A role with the same title as a built-in one overrides it; otherwise
it is appended as a new role. No mapping files or config changes needed.

Changes

File	Change	Description
role.py	+30 lines	Added roles/ directory scan in __init__ and new _load_role_file method
README.md	Updated	Documented the new roles/ directory usage

Function	Role
_load_role_file(filepath)	Load a single .json role file; if title matches an existing role, evict it from
old tag lists and replace; otherwise append as new. Unknown tags are auto-registered.
__init__ (modified)	After loading built-in roles.json, scan roles/ for *.json files and merge them via
_load_role_file. Clean up empty tags afterward.

Type of change

• Bug fix
• New feature
• Docs
• Refactor / chore

Checklist

• I have read the Contributing Guide
• I tested this change locally
• Code comments and docs are in English
• Linked related issue (if any): closes #

[zhayujie]

thanks for the quick turnaround, this is exactly the right approach! Keeping roles.json untouched and scanning an optional roles/ directory makes it fully backward compatible: default installs are unaffected (the os.path.isdir guard skips when the dir is absent), and per-file load failures are caught so they can't break plugin init. Clean and minimal. Merging now
thanks for the contribution 🎉