Track a standalone ready-for-agent issue as a plan

[JBlaak]

An issue labelled ready-for-agent with no parent and no sub-issues is now a plan with itself as its one issue. It shows in the plan picker, opens as a one-node plan with its blockers and PRs, and is the plan of a session whose branch or PR points at it. A labelled issue under a parent stays in its parent's plan only.

• The picker's search is now (has:sub-issue OR (label:ready-for-agent no:parent-issue)).
• parentsOf is renamed to plansOf. For each issue it gives the parent, or the issue itself when it is standalone and labelled.
• Checked against JBlaak/James: Add a light mode with a system/light/dark toggle #23 shows next to Add global terminal font, size, and line height settings #1 and Remove the unused session buttons, sidebar tabs, grid and IDE emulation #22 as a plan of one, and Drop the short session id from the session list #102 still leads to its parent Leave PRs from other repositories off the Session work page #98.

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