Default to OpenCode and clean attach returns

[aksOps]

Summary

• suppress UAM-returning attach detach notices so the shell stays clean after leaving the dashboard
• default new configs and TUI fallback to OpenCode when available
• make provider [claude]: command alias [default]: workdir [/home/dev/projects/unified-agent-manager]: prompt [#session-name prompt, optional]: dispatched 561afa21-6c16-4b0c-b720-f220f5bd6acb (uam-claude-561afa21) attach to the newly created session immediately

Test Plan

• go test ./internal/session ./internal/app ./internal/cli ./internal/store -count=1
• go test ./... -count=1

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
92.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud