feat: normalize markdown message send/reply output

[91-enjoy]

Summary

• clean up markdown post conversion output for IM send/reply
• update tests for the new markdown normalization behavior
• align Skills docs with the implementation

What changed

• +messages-send and +messages-reply no longer inject literal <br> into markdown-derived post content
• markdown spacing is normalized with blank lines, and fenced code blocks are preserved without extra wrappers
• the send/reply Skills docs were updated to match the current --markdown behavior

Test plan

• run env GOCACHE=/tmp/go-build-cache go test ./...
• verify the markdown conversion tests cover the updated heading, spacing, and code block output
• confirm the send/reply Skills describe markdown as normalized post conversion rather than raw markdown passthrough

[CLAassistant]

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