[FEATURE]: Add a new experimental "hashline" edit mode

[watzon]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Hashline is a new edit mode just introduced to the oh-my-pi agentic coding tool (a fork of Pi). In a lot of cases it speeds up edits and reduces rejected edits from diffs not matching. Having tested it out myself in oh-my-pi I really want to bring the feature over to OpenCode.

References:

• https://blog.can.ac/2026/02/12/the-harness-problem/
• https://github.com/can1357/oh-my-pi

[github-actions]

Thanks for the feature request! Before implementation, please check if this might relate to existing issues addressing similar edit tool challenges:

• [FEATURE]: improve edit tool validation to use checksum of content in addition to timestamp #12406: Proposes using checksums for file verification (similar solution to hashline)
• [Tracking] Edit tool reliability: "modified since last read" errors (Undo/Redo & Persistence) #5840: Tracking issue for edit tool reliability, includes discussion of content-based hashing
• Why must the read tool be executed before the edit tool #4406: Discusses the underlying need to re-read files between edits

These issues explore similar solutions to the 'rejected edits' problem that hashline addresses. Linking them might help the team prioritize and coordinate implementation.