docs: add out-of-scope entry for hashline content-hash anchors

Records the prior rejection of content-hash line anchors for LLM edits
(#141). Pre-existing KB entry that was never committed.

.out-of-scope/hashline.md

@@ -0,0 +1,28 @@
+# Hashline (content-hash line anchors for LLM edits)
+
+This project does not implement hashline / content-hash line anchoring for LLM edits.
+
+## Why this is out of scope
+
+Hashline is a technique where each line returned by the `read` tool is prefixed
+with a short content-hash anchor (e.g. `9#KT:  console.log(...)`), and the LLM
+references edits by `LINE#HASH` anchor instead of quoting raw text. The system
+validates the hash before applying an edit, so if the file changed between read
+and edit the hash mismatches and the edit is rejected before it can corrupt
+anything. Hashes are context-based (`xxh32(prev + curr + next)` over a 16-char
+alphabet), so editing line N only invalidates N-1/N/N+1.
+
+Implementing it requires wrapping OpenCode's core `read` and `edit` tools to
+inject and validate anchors and track file snapshots for stale-anchor recovery.
+That is a deep, behavior-changing modification to the fundamental edit loop —
+fragile to bolt onto a slim plugin that intentionally avoids reimplementing tool
+plumbing. It belongs in OpenCode core itself or a dedicated standalone plugin,
+not in oh-my-opencode-slim.
+
+Token savings are real (reported ~61% fewer output tokens on Grok 4 Fast, ~8%
+better on Gemini), but the integration cost and architectural fit put it
+outside this project's scope.
+
+## Prior requests
+
+- #141 — "Discussion about hashline" (feature proposal / discussion; closed as wontfix)