feat: add Brainstorm Mode toggle with divergent ideation prompts

[jadeftx]

Closes #2

Summary

• Adds Brainstorm Mode toggle in Settings that flips the entire prompt engine from convergent dialectic to divergent ideation
• Settings dropdowns swap dynamically: Length→Expansion (Sparks/Seeds/Branches), Tone→Perspective (Rapid-Fire/Contrarian/First Principles)
• New Ideation Vector dropdown (Who/What/When/Where/Why/How/What-If/ALL)
• Brainstorm-specific prompt frames with tangibility constraint and Gemini filler suppression
• Idea coordinate numbering [R#-#] in side panel rendering, markdown export, and JSON export
• UI adapts across the board: header subtitle, consensus button text, confirm dialogs
• Per-mode modifier preferences preserved independently when switching modes

Files changed (6 files, +338 −43)

• sidepanel/prompts.js — brainstorm constants + branched prompt builders
• sidepanel/sidepanel.js — toggle wiring, dynamic dropdown swap, coordinate rendering, export enrichment
• sidepanel/sidepanel.html — Mode toggle section + Ideation Vector dropdown
• sidepanel/sidepanel.css — .idea-coord style
• background.js — coordinate annotation in GitHub push markdown export
• manifest.json — version bump to 1.1.0

Test plan

• Toggle Brainstorm ON → verify dropdown labels/options swap, vector appears
• Toggle OFF → verify labels/options revert, vector hides, dialectic preferences preserved
• Start brainstorm session → verify === BRAINSTORM — ROUND 1 === in prompt
• Handoff to Gemini → verify filler suppression line appended
• Verify [R#-#] coordinates appear in side panel on Rapid-Fire idea lines
• Export markdown → verify [R#-#] prepended to idea lines
• Export JSON → verify ideas array with ideaIndex fields on AI messages
• Consensus button shows "End Brainstorm" / "Reopen Brainstorm"