feat: add 18 Cursor-like features to chatbot (regenerate, edit, diff, search, slash commands, pinned files, export, Ctrl+K)

[Copilot]

Extends the existing Claude Code chat panel with the core features that make Cursor feel "pro" — covering message editing/regeneration, diff-before-apply, context persistence, conversation tooling, and editor integration.

High Impact

• Regenerate ↺ — _regenPrompt stored on each assistant div; regenerateFrom() trims sessionMessages and re-sends
• Edit user message ✏ — pencil button on hover; inline textarea replaces bubble, truncates session history, re-sends on confirm
• Inline diff view — computeDiff() (LCS/O(nm)) + buildDiffView() replace the plain code preview in the apply modal; diff data arrives via new getActiveFileContent ↔ activeFileContent round-trip
• Drag-and-drop files — dragover/drop on #input-wrapper; filters by code extension
• Up-arrow recall — empty input + ↑ fills textarea with lastUserMessage

Medium Impact

• Pinned context files — 📎 per chip; stored in openClaudeCode.pinnedFiles config; restored automatically on New Chat and sessionCleared
• Export as Markdown — header button serializes sessionMessages → exportConversation message → VS Code save dialog
• Context window usage bar — MODEL_CONTEXT map keyed by model id; updateContextBar() called on every tokenUpdate; hidden until tokens > 0
• History rename / delete — inline prompt()/confirm() → renameSession / deleteSession handlers in extension, globalState updated, list refreshed
• Ctrl+F search — highlights matching .msg-user / .msg-assistant divs; prev/next scroll; Esc closes
• Auto-scroll lock — autoScroll flag guards scrollToBottom(); header button toggles; re-enables automatically when user scrolls to bottom
• Slash commands — /clear /model /export /help /pin reuse the existing #autocomplete dropdown and showAutocomplete/hideAutocomplete path

Lower Impact / Editor Integration

• Image paste — paste event → FileReader.readAsDataURL → chip (data URI validated against /^data:image\/[a-z]+;base64,/ before img.src assignment)
• @codebase mention — input handler replaces token with a chip; useCodebase flag forwarded in send message
• Ctrl+K inline edit — new openClaudeCode.inlineEdit command reads editor selection + filename, focuses panel, sends inlineEditRequest; handler pre-fills input with fenced code + instruction prompt
• Keybindings — Ctrl+Alt+C focuses panel; Ctrl+K (when editorTextFocus) triggers inline edit
• openClaudeCode.pinnedFiles config property added to package.json contributions