Remove redundant WithEmitTaskFrontmatter option - always emit frontmatter

[Copilot]

The WithEmitTaskFrontmatter option and -t flag were redundant. Task frontmatter contains metadata needed by AI agents and downstream tools, so there's no use case for omitting it.

Changes

• Removed -t CLI flag - frontmatter now always emitted when present
• Removed WithEmitTaskFrontmatter option from Context configuration API
• Removed emitTaskFrontmatter field from Context struct
• Updated integration test TestPrintTaskFrontmatter to verify default behavior
• Updated documentation to reflect automatic frontmatter emission

Before/After

# Before: required -t flag
coding-context -t implement-feature

# After: always included
coding-context implement-feature

Both produce the same output with frontmatter at the beginning.

Original prompt

WithEmitTaskFrontmatter seems redundant. Remove it. Default to true.

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