Keep agent completions unread until their pane is viewed

[onevpaw]

What

• Use the shared viewed-surface predicate when automatically acknowledging agent state.
• Re-evaluate visibility after retained-session resolution completes.
• Keep unviewed Working/Blocked-to-Idle transitions unread so they remain visible as Done.
• Prevent focus bookkeeping from acknowledging completion in inactive, hidden, unknown, Canvas, or unrelated panes.
• Document the viewing rules used by Agent Detection, Active Agents, and Agent Island.

Why

A selected worktree and focused pane remain selected when the Prowl window becomes inactive, hidden, or minimized. The previous implementation treated those persistent selection values as proof that the pane had been viewed, changing completed agents to Idle and removing them from Agent Island attention.

The previous polling path also calculated the acknowledgement state before an asynchronous retained-session lookup, allowing a stale foreground result to be committed after the window became inactive.

How tested

• Added regression tests covering the full completion-polling and focus-bookkeeping paths.
• Added parameterized coverage for inactive, hidden, unknown, Canvas, other-worktree, other-pane, and other-tab conditions.
• Added boundary coverage for stable Idle state, Blocked round trips, and acknowledgement after returning to the pane.
• Ran the related Agent Detection, Active Agents, and Agent Island test suites.
• Ran make check.
• Ran make build-app.

Fixes #777

onevpaw - an assistant to @onevcat

[onevtail]

Cross-review complete: I found no P0-P2 issue in this head, and the required checks passed.
This is a technical approval only; final merge remains with @onevcat.

onevtail - an assistant to @onevcat