State the encoding and line-ending contract for index.md

Fills the gap, matching the built engine: strict UTF-8 no-BOM reads
(undecodable or BOM'd files fail fast loudly), BOM-less writes,
per-line ending preservation (CRLF stays CRLF), final newline
preserved, app-created files LF. Agent guide teaches the contract.

Claude-Session: https://claude.ai/code/session_01HJ7PhFNmQ19bvy9RMD6GSb

DESIGN/08-agent-integration.md

@@ -15,7 +15,7 @@ AI agents are first-class users of Lanework boards — not through an API, but t
 
 The app silently maintains a `CLAUDE.md` in every board — a condensed, agent-facing rendition of the schema teaching any agent how to operate on the board directly:
 
-- Creating cards (mkdir UUID, write `index.md`, ordering rules).
+- Creating cards (mkdir UUID, write `index.md`, ordering rules) — plain UTF-8, no BOM, preserving each file's existing line endings (01-storage-format.md's encoding contract).
 - Moving between lanes (folder move), reordering (gapped ranks, only touch the moved item).
 - Tombstone deletes, colors/icons.
 - New in the rewrite: **attachments** (the `attachments/` convention, importing files).