fix(coding-agent): reject ctx.ui.custom() under daemon/RPC instead of silent no-op

[pushpraj-core]

Summary

Fixes #680

ctx.ui.custom() was silently no-opping under the daemon/worker architecture and over RPC, returning undefined instead of surfacing an error. This made it impossible for extensions to detect that terminal-takeover rendering wasn't available.

Changes

• ExtensionUIContext.supportsCustom (new property): a readonly boolean flag that extensions can check before calling custom() to detect whether terminal-takeover rendering is supported in the current UI context.
• noOpUIContext (runner.ts): custom() now throws instead of returning undefined as never.
• Daemon binding (daemon-extension-binding.ts): custom() now throws with a descriptive error explaining the daemon/worker limitation.
• RPC bridge (rpc-extension-ui-context.ts): custom() now throws with a descriptive error explaining the RPC limitation.
• Interactive mode (interactive-mode.ts): sets supportsCustom: true since it owns a real terminal.
• CHANGELOG: documents the fix.

How to test

1. Call ctx.ui.custom() from an extension running under the daemon — it should now throw instead of silently resolving.
2. Check ctx.ui.supportsCustom — it should be false under daemon/RPC and true in interactive mode.
3. Type-check: npx tsgo --noEmit -p tsconfig.build.json

Note

Fix ctx.ui.custom() to throw instead of silently no-op in daemon and RPC modes

• Adds a supportsCustom: boolean flag to the ExtensionUIContext interface so callers can check whether terminal takeover is available before calling custom().
• In daemon/worker and RPC modes, ctx.ui.custom() now throws an Error instead of silently resolving; interactive mode sets supportsCustom: true.
• Behavioral Change: extensions that previously relied on custom() silently no-oping in non-interactive contexts will now receive a thrown error.

^{Macroscope summarized c9981c4.}