[FEATURE]:feat: support selective acceptance of code changes (per-file/per-hunk) instead of all-or-nothing

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Problem

Currently opencode applies all code changes unconditionally. If a specific line or file edit is incorrect, the user has no way to reject only that change — it is all or nothing.

Expected Behavior

Provide a mechanism for selective acceptance:

1. Per-file accept/reject: Show a list of modified files, let the user choose which to keep
2. Per-hunk/per-line accept/reject: Interactive mode similar to git add -p, allowing the user to review and accept/reject each hunk
3. Diff preview + confirmation: Show the diff before writing, ask for confirmation

Use Cases

• Most edits in a file are correct but one line has wrong logic
• A particular file change violates project conventions
• User wants to adopt only some suggestions while keeping the rest original