feat: add Visual Conflict Resolver, Semantic Mermaid Diff, and v1.5.0 updates

[phine-apps]

Summary

This PR prepares the v1.5.0 release with major feature additions, documentation improvements, and regression testing updates:

1. Visual Conflict Resolver (CustomTextEditorProvider):

   • Interactive 3-way visual merge editor for resolving Git merge conflicts (<<<<<<<, =======, >>>>>>>) directly in rendered Markdown with one-click actions (Accept Mine, Accept Theirs, Accept Both).
   • Registered under customEditors as rich-markdown-diff.conflictResolver.

2. Semantic Mermaid Diff:

   • Element-level diff highlighting for Mermaid flowcharts.
   • Dynamically color-codes added/modified/deleted nodes and edges with ghost outlines for deleted elements instead of falling back to full block replacement.

3. Documentation & Release Updates:

   • Updated CHANGELOG.md with concise release notes for v1.5.0.
   • Updated README.md with usage instructions for HTML export, committed revision diffs, and a default keyboard shortcuts reference.
   • Bumped package.json version to 1.5.0.

Test Plan

• All 212 unit tests pass (pnpm run test:unit)
• Visual conflict parser tests pass (src/test/unit/conflictParser.test.ts)
• Mermaid semantic diff tests pass (src/test/unit/mermaidDiff.test.ts)
• Extension integration tests and lint checks pass (pnpm run pretest)