[WIP] Add support for variable-aware dynamic prompt templates

[Claude]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>[Feature] Variable-aware Dynamic Prompt Templates</issue_title>
<issue_description>Currently, templates in cjules (~/.config/cjules/templates/) are static text. It would be incredibly powerful to support dynamic prompt rendering with variables and local context, such as current git diffs or file contents.

Example Usage:
A template refactor.md:

Please refactor the following code:

[Code]
{{.File "src/parser.cr"}}

[Notes]
{{.Var "note"}}

Command:

cjules new --template refactor --var note="Improve error handling"

Proposed variables/helpers to support:

• {{.File "relative/path"}} : Inserts the content of the specified file.
• {{.GitDiff}} : Inserts the current git diff of the repository.
• {{.Var "name"}} : Inserts a user-defined variable passed via --var name=value.</issue_description>

Comments on the Issue (you are @claude[agent] in this section)

[hahwul]

@claude[agent] /goal Let's complete this function properly.

[Claude]

@claude[agent] /goal Let's complete this function properly.

The feature implementation is complete and fully functional. All three template helpers ({{.File}}, {{.GitDiff}}, {{.Var}}) are working with proper error handling, the --var flag is available in both new and retry commands, and all 85 tests pass.

The PR description has been updated with the full implementation details. Ready for review when you approve the CI workflow run.

[Copilot]

Pull request overview

Adds a lightweight template rendering engine to cjules so prompt templates can be dynamically expanded using local context (file contents, git diff, and user-provided variables), and wires --var KEY=VALUE through new/retry with updated docs and specs.

Changes:

• Introduces Cjules::TemplateRenderer with {{.File "path"}}, {{.GitDiff}}, and {{.Var "name"}} directives.
• Adds --var KEY=VALUE support to cjules new and cjules retry, and threads variables into prompt resolution.
• Updates CLI help + README, and adds specs for template parsing/rendering.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/cjules/util.cr	Extends prompt resolution to optionally apply template rendering.
src/cjules/template_renderer.cr	New renderer implementing .File, .GitDiff, .Var directives + --var parsing.
src/cjules/commands/templates.cr	Documents the new template directives in help text.
src/cjules/commands/retry.cr	Adds --var flag and applies rendering to prompt overrides/files.
src/cjules/commands/new.cr	Adds --var flag and passes variables into prompt resolution.
spec/cjules_spec.cr	Adds unit specs for TemplateRenderer.parse_vars and .render.
README.md	Documents dynamic template directives and usage examples.

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