fix(coding-agent): respect startup.quiet for startup statuses

[roboomp]

Repro

With startup.quiet=true, emitting mcp:connecting still reached InteractiveMode.showStatus, and emitting a failed lsp:startup event still reached InteractiveMode.showWarning. I reproduced this with cd packages/coding-agent && bun test test/interactive-mode-mcp-connecting.test.ts test/interactive-mode-lsp-startup.test.ts, which failed before the fix with 3 pass, 2 fail.

Cause

packages/coding-agent/src/sdk.ts emitted MCP_CONNECTING_EVENT_CHANNEL and LSP_STARTUP_EVENT_CHANNEL whenever UI was available, without checking settings.get("startup.quiet"); packages/coding-agent/src/modes/interactive-mode.ts also rendered those events without consulting the same setting.

Fix

• Gate MCP connecting emits in createAgentSession when startup.quiet is enabled.
• Gate LSP startup completion/failure emits in createAgentSession when startup.quiet is enabled.
• Add matching InteractiveMode subscriber guards so externally emitted startup events stay quiet.
• Add regression coverage for both MCP connecting and LSP startup event paths.

Verification

cd packages/coding-agent && bun test test/interactive-mode-mcp-connecting.test.ts test/interactive-mode-lsp-startup.test.ts passed with 5 pass, 0 fail; cd packages/coding-agent && bun run check passed. Fixes #2639