docs: improve channel README with app URL

[ai-cora]

Summary

• Removed manual .mcp.json config section -- the server ships with the VoiceMode plugin
• Added "Make a call" section linking to app.voicemode.dev
• Cleaner, shorter README that tells users what they actually need to do

🤖 Generated with Claude Code

[JiwaniZakir]

The restructuring of the unreleased section drops the dedicated #### Security heading and folds what was a standalone security callout into a sub-bullet (Explicit opt-in required: set VOICEMODE_CHANNEL_ENABLED=true). Security-relevant behaviors generally benefit from being visually distinct in a changelog — a user scanning for "what changed" is more likely to notice a #### Security heading than a sixth nested bullet under a feature entry. Consider keeping the security opt-in as its own subsection or at minimum promoting it to a top-level bullet rather than nesting it after implementation details.

The diff also includes a large number of blank-line-only insertions after ### Added, ### Fixed, ### Changed, etc. headings across historical entries (e.g., 6.1.1, 6.0.x, 5.1.x). These are purely cosmetic and create significant noise in the diff, making it harder to identify the substantive changes. If normalizing whitespace throughout the file, that cleanup is better done in a separate, dedicated commit so the structural content changes here remain easy to review and revert independently.