core/tui: MCP startup/list concurrency caps + TUI exec live-tail

Problem

• MCP server startup storms and timeouts on weak/unstable environments.
• No visible progress on long exec; TurnDiff logs are verbose.

Proposal

• Cap MCP server start concurrency (3) with a quick backoff retry (~750ms) on failure.
• Cap tools/list concurrency (4).
• TUI: show live tail of ExecCommandOutputDelta in the status line; log concise TurnDiff summary (bytes/lines) instead of full diff.

Expected Impact

• Fewer timeouts at startup and calmer logs.
• Better UX for long‑running commands.

Compatibility

• Internal behavioral changes only; protocol unchanged.

Risks

• Caps may need tuning; retry could mask longer transient issues (parameters are conservative).