feat: allow users to edit suggested changes before applying

[dmmulroy]

Adds ability for users to edit suggested file changes in their editor before applying them.

When the edit tool proposes changes, users can now press e to open the suggested content in their editor, make modifications, and have those modifications applied instead of the original suggestion.

Changes

• packages/opencode/src/permission/editor.ts - new module for computing diffs between original and user-modified content
• packages/opencode/src/util/text.ts - shared text utilities (normalizeLineEndings, getFirstDifferingLine, hasChanges)
• packages/opencode/src/cli/cmd/tui/util/editor.ts - Editor.Result type for better error handling
• packages/opencode/src/server/server.ts - typed modifyData validation
• packages/opencode/src/cli/cmd/tui/routes/session/index.tsx - UI integration for edit flow

Scope

• Edit tool (single-file): supported
• Patch tool (multi-file): unchanged, to be added separately in the future if desired

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This PR was created with AI assistance

[allisoneer]

This is really really really good. I just want to +1 the directional intentions of these changes.