Improve repository discovery metadata

[Suren878]

Summary

• Update README first-screen copy around local-first AI assistant/runtime keywords.
• Add common use cases for Terminal, Telegram, MCP, providers, and subagents.
• Align feature bullets with Codex and Claude Code external agents.

GitHub Metadata

• Updated repository description.
• Added discovery topics for AI assistant, local-first, subagents, MCP, Codex, Claude Code, and related categories.

Test Plan

• git diff --check
• go vet ./...
• go test ./...
• /root/go/bin/golangci-lint run --new-from-rev=origin/main