Replace text/template with os.Expand() for variable substitution

[Copilot]

Replaces Go's text/template package with os.Expand() for simpler, shell-style variable expansion in prompt files.

Changes

Code (main.go)

• Removed text/template import and template parsing/execution logic
• Implemented variable substitution using os.Expand() with a mapping function
• Missing parameters now expand to empty strings (vs. <no value>)

Syntax

• Before: {{ .variableName }}
• After: ${variableName} or $variableName

Tests (integration_test.go)

• Added TestTemplateExpansionWithOsExpand for both brace and non-brace syntax
• Added TestTemplateExpansionWithMissingParams for empty string behavior

Documentation (README.md)

• Updated all examples and documentation to use shell-style expansion syntax

Example

# Task: ${taskName}

Implement ${feature} in ${language}.
Environment: $env

coding-context -p taskName="AddAuth" -p feature="OAuth" -p language="Go" -p env="prod" my-task

Original prompt

Change to use os.Expand() instead of text/template.

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