feat(notifications): name the session a notification is about

[Jonathan-Asher]

Summary

OS notifications raised by agent sessions (turn finished or failed, session error, permission request, question, background task settled) were titled <folder> - Codeg with the window's active folder, not the folder of the session that raised them, and the body named only the agent. With several sessions running there was no way to tell which one needed you, and the folder in the title could be the wrong one.

Each of these notifications now names its session: the session's title as the notification title, and the session's own folder at the start of the body.

Behavior

• Title: the conversation's title, with reference links folded the same way as in the tab strip. A draft that has no conversation row yet uses its tab label.
• Body: <folder> · <message>, where the folder is the session's own (its alias when one is set). A chat-mode conversation names its hidden "Chat" folder; a chat draft that has no folder yet names none rather than the active one.
• "Hide notification contents" on: the notification keeps its previous shape, <folder> - Codeg over the message alone, but names the session's folder. A session title is the user's own words (often the first line of their prompt), which that setting exists to keep out of the notification centre.
• A connection no tab owns (a canvas card, a delegated sub-agent's own connection) keeps the previous <active folder> - Codeg title.
• Work-task notifications from the Tasks view are unchanged; they already name the task's own folder.

Implementation:

• src/lib/notification-session.ts (new): sessionNotificationPayload(contextKey, activeFolderName, { body, redactedBody }). The connection's context key is its tab id; from the tab it finds the persisted conversation (through the runtime session's row id for a draft whose first send has not bound the tab yet) and the folder in allFolders.
• src/lib/desktop-notification.ts: NotifyPayload gains an optional redactedTitle, used when contents are hidden, the same way as redactedBody.
• src/contexts/acp-connections-context.tsx: the six notifyDesktop call sites wrap their existing payloads in sessionNotificationPayload. What they notify and when (failure-aware turn completion, acpErrorNotifiesDesktop, quiet) is unchanged.

Verification

• src/lib/notification-session.test.ts (new, 7 tests): the session title and the session's own folder win over a different active folder; folder alias and reference-link folding; draft tab label; a draft reached through its runtime session; a chat-mode conversation's hidden folder; a folderless chat draft names no folder; an unknown context key keeps the old title.
• src/lib/desktop-notification.test.ts: the redacted title replaces the title only when contents are hidden.
• CI passes: the frontend job (pnpm lint ., pnpm test, pnpm build) and every Rust desktop/server cell on Ubuntu, macOS and Windows. There are no Rust changes.

[Jonathan-Asher]

Opened upstream as xintaofei#834.