Add continued agent conversations

[klopez4212]

Summary

• Add channel-scoped continued agent conversations with sidebar entries and dedicated conversation markers.
• Keep source threads intact while follow-up continues in the dedicated conversation view, including recap-to-thread and shimmer thinking states.
• Tighten one-agent auto-routing and refresh managed-agent auth/command fallback for staging identity reliability.

Tests

• cargo fmt --all -- --check
• cargo test -p buzz-acp
• cargo test -p buzz-core
• cargo test --manifest-path desktop/src-tauri/Cargo.toml managed_agents::runtime::tests::build_env --lib
• cargo test --manifest-path desktop/src-tauri/Cargo.toml managed_agents::discovery::tests::effective_agent_command --lib
• cd desktop && ./node_modules/.bin/tsc --noEmit --pretty false
• cd desktop && ../node_modules/.bin/biome check . (passes with existing CSS specificity warnings)
• cd desktop && node ./scripts/check-file-sizes.mjs && node ./scripts/check-px-text.mjs
• cd desktop && node --import ./test-loader.mjs --experimental-strip-types --test src/features/agents/agentConversations.test.mjs src/features/channels/ui/ChannelPane.helpers.test.mjs src/features/messages/lib/threadPanel.test.mjs