Spaces overview: ranking + metadata for connector-backed spaces

[spaceshipmike]

Context: Surfaced while drafting the alpha outreach email (#24). The Spaces overview is going to be one of the first things a tester sees after first-time save flow — and right now it leads with auto-created connector spaces (one per connector instance) that render as a bare list of links with no metadata.

The problems:

1. Ranking — connector-backed spaces (which exist because §2.0a auto-creates a backing space per connector instance) often have the largest member counts and most recent activity, so any sort that defaults to recency or member_count surfaces them first. They're not the spaces a tester cares about; their Projects and topical Spaces are.
2. Metadata — the connector-space detail view renders a list of titles + URLs without showing the things that distinguish spaces: rolling activity (recent saves, last-touched date), brief summary, dominant content types, source-domain mix, the connector instance metadata.
3. First-impression effect — a tester who lands on a connector space first reads Knowmarks as "a bookmark list with extra steps" rather than "the AI-readable knowledge layer."

Proposed direction (sketch — agent picks impl):

• Default sort on /spaces should de-prioritize connector-backed spaces (push them below Project + Topical + Curated, or hide them behind an "Imports" filter chip).
• Connector-space detail view should grow a header that mirrors what other spaces show: brief, recent activity, content-type mix, freshness signal. The connector linkage should be visible ("Synced from GitHub stars · last sync 4h ago · 1,247 items") but not be the whole UI.
• Auto-created connector spaces should default to a less prominent lifetime_intent or a UI-only collapsed state until the user pins them.

Acceptance:

• A new tester landing on /spaces first sees the spaces relevant to their thinking (Projects, then topical Spaces), not the bulk-imported connector spaces.
• Opening a connector space shows enough metadata to understand what it is and how it's being kept fresh.
• Sort + filter affordances let a power user surface connector spaces explicitly when they want to.

Related: #24 (alpha master list).