feat(config): accept ${env.X} in path fields (steps 2-4 of #2615)

[dgageot]

docker-agent has two incompatible environment variable expansion syntaxes: JavaScript template literals (${env.X}) for prompt, header, and command fields, and shell-style ($VAR, ${VAR}, ~) for path fields. Using the wrong syntax in a path field was silently ignored, tracked in #2615.

This PR makes path fields accept both syntaxes. pkg/path.ExpandPath now normalizes the JavaScript form ${env.VAR} to ${VAR} before calling os.ExpandEnv, so toolset working_dir, path fields (memory and tasks path, mcp and lsp working_dir), and related fields accept either syntax. The change is fully backward-compatible — existing $VAR/${VAR}/~ configurations are unchanged.

Only plain ${env.VAR} references are supported in paths; richer expressions like ${env.X || 'default'} are not evaluated (would require importing the goja engine, creating a circular dependency). Resolution uses the OS environment, matching the existing ${X} behavior. The now-obsolete mismatch warnings for working_dir and path fields have been removed from pkg/config/expansion_warnings.go. Warnings remain for truly shell-only fields: toolset env values, ScriptShell working_dir/env, and hook working_dir/env.

This is a partial fix for #2615; a follow-up will address the remaining shell-only fields in step 5. Documentation has been updated with a new variable-expansion matrix and cross-reference comments on the duplicated regex.

Refs #2615

[docker-agent]

Assessment: 🟢 APPROVE

The diff was fully reviewed. No bugs or logic errors were found in the changed code.

Summary of changes reviewed:

• pkg/path/expand.go: New ${env.VAR} → ${VAR} normalization via regex substitution before os.ExpandEnv. The regex and substitution logic are correct; backward-compatibility with existing $VAR/${VAR}/~ is preserved.
• pkg/config/expansion_warnings.go: Removal of now-obsolete mismatch warnings for path fields is appropriate given the new dual-syntax support.
• pkg/path/expand_test.go / pkg/config/expansion_warnings_test.go: Test coverage for the new behaviour and updated warning set looks complete.
• Documentation updates accurately describe the new dual-syntax support and variable-expansion matrix.

No issues found.