tabs: persist a seenAt stamp per URL so an agent can tell which tabs are new since it last looked

[colevels]

Problem

An agent calling tabbrew tabs repeatedly cannot tell which tabs it has already been shown and which are new since its last look. lastAccessed (already on TabSnapshot) answers a different question: when the user last focused the tab, not when the CLI last reported it.

Proposal

Add seenAt?: number to tab output: the time the CLI previously returned a tab with this URL. Absent means never seen, same convention as lastAccessed (absent is not zero).

Owned by the CLI session server, not the extension

• Only the CLI knows what it actually showed the caller. The extension only knows it answered an operator call.
• The harness and Web Store extensions have different ids, so chrome.storage would be split between them and lost on reinstall.
• Storing it in the extension would need a new write-back operator for data the CLI already owns. The extension stays a stateless pass-through of Chrome APIs.
• Precedent: createWindowLabels() in src/core/session/labels.ts already keeps a session-side map and stamps every output that passes through the server.

Keyed by URL

Chrome tab ids do not survive a browser restart, so URL is the only key that works across sessions. URL alone is enough, no parallel tabId map:

• A tab that navigates to a new URL reads as unseen, which is correct (new content).
• Two tabs on the same URL share one seenAt, which matches "this page has been seen".
• Strip #fragment before keying. Keep the query string, since many sites use it to distinguish pages.

Stored in ~/.tabbrew/seen.json, single writer

• STATE_DIR already exists in src/core/session/config.ts.
• The session server is the only writer: load on start, stamp the previous seenAt onto the output, update memory, then flush via temp file + rename. Individual CLI processes must not write the file, or concurrent calls would clobber each other.
• The server exits after IDLE_EXIT_MS, so flush on change (or a short debounce), not on exit.
• A corrupt or unreadable file reads as empty, like parse() in src/core/session/project.ts. It must never keep a session from starting.

Open questions

1. Privacy: the file is effectively browsing history on disk. Mode 0600, removed by uninstall (check whether uninstall already removes all of STATE_DIR), and incognito windows skipped.
2. Size: prune by age (e.g. entries older than 90 days) or cap the entry count.
3. Multiple Chrome profiles: shared or separate? Suggest shared to start, since the question is "has the agent seen this URL", which does not depend on the profile.

Implementation sketch

• src/core/session/seen.ts, following the shape of createWindowLabels(), with tests.
• Wire into the same stamping path in src/core/session/server.ts.
• Surface seenAt in src/core/tabs/format.ts.