feat(node-summary): single-click opens summary sheet, double-click opens editor

[SobshDev]

Summary

• Single-clicking a file node opens a summary sheet with an AI-generated overview and lists of Uses / Used by files.
• Summaries are cached to .graphy/summaries.json keyed by file path, with a fingerprint of source hash + sorted caller/callee file paths so cosmetic edits in unrelated files don't invalidate.
• Double-click now opens the code editor (was single-click).

Test plan

• Single-click a file node → summary sheet opens, "Uses" / "Used by" populated.
• Sheet shows "Generating summary…" then prose once the AI returns.
• Re-click the same node → summary loads from cache instantly (no streaming state).
• Edit the file source, click again → summary regenerates.
• Click Refresh in the sheet header → forces regeneration even when cache is warm.
• Double-click a file node → code editor sheet opens (existing behavior, just remapped).
• With no AI key configured, sheet shows the "configure an AI API key" CTA.