Allow TUI plugins to suppress attention notifications for internal sessions

Problem

Plugins like opencode-mem create internal background sessions (titled "opencode-mem capture") for structured-output LLM calls. These are top-level sessions (no parentID), so the builtin internal:notifications TUI plugin fires desktop notifications and sounds every time one completes — even though the user never interacted with that session.

Currently there's no way to tell the notifications plugin to skip certain sessions. The only workaround is for a plugin to deactivate("internal:notifications") and re-implement the entire notification logic with a filter, which means the plugin won't benefit from upstream improvements to the notification logic.

Proposed solution

One or more of:

1. Config-based filter: Add a tui.json option like attention.excludeTitles: ["opencode-mem capture"] or attention.excludeMetadata: { "opencode-mem.internal": true } that the builtin notifications plugin checks before firing.

2. API hook: Expose a api.attention.shouldNotify(sessionID) hook or api.attention.filter(predicate) method that plugins can use to register exclusion rules.

3. Session metadata convention: The builtin notifications plugin could check session.metadata for a notifications: false or internal: true flag and skip those sessions. opencode-mem already sets metadata: { "opencode-mem": { internal: true, purpose: "structured-output" } } on its capture sessions — the notifications plugin just doesn't look at it.

Option 3 seems lowest-effort since the metadata is already there.

Context

• opencode-mem sets metadata["opencode-mem"].internal = true on its capture sessions: internal-capture-sessions.js
• The builtin notifications plugin fires on session.status → idle for any session that was previously busy, with no metadata check: notifications.ts