path template updates

[rsgalloway]

Path templating: decouple from TEMPLATES_ENV_NAME, support arbitrary envs, and expand ${VARS} before {fields}

Summary

The current path templating implementation in path.py is overly coupled to a hard-coded TEMPLATES_ENV_NAME (templates.env) and does not support resolving ${ENV_VARS} (e.g. ${ROOT}) before applying {field} token substitution. This limits flexibility for tool-specific and project-specific path templates and breaks common real-world use cases.

• This issue proposes generalizing path templating so that:
• Templates can live in any envstack environment (tool-defined, project-scoped, etc.)
• ${VAR} expansion is performed using envstack’s resolved environment before {field} formatting
• The concept of a special “templates env” is deprecated or removed
• Issue trailing curly braces get trimmed #105 already tracks the trailing } truncation bug

Use Case

• Different tools may want their own path template namespaces (e.g. maya.env, nuke.env)
• Projects may override or extend default templates using envstack’s existing layering model
• Common templates require ${ROOT}, ${SHOW}, etc. to be resolved prior to {seq}, {shot}, {version} formatting
• Path templating should be a generic capability, not a special-cased env name

Example (currently broken):

NUKESCRIPT: ${ROOT}/projects/{seq}/{shot}/comp/{show}_{seq}_{shot}.{version}.nk

[rsgalloway]

Instead of get_template(name) implicitly loading templates.env, make it explicit:

get_template(name, stack="nuke", scope=...)