[DOCS/FEATURE] MEMORY.md has undocumented 200-line hard limit

Problem

The auto-memory feature (MEMORY.md) has a 200-line hard limit that is not documented anywhere. Lines beyond 200 are silently truncated when loaded into context.

Current Behavior

• Claude Code creates/updates ~/.claude/projects/<path>/memory/MEMORY.md automatically
• The file can contain any number of lines
• Only the first 200 lines are loaded into context at session start
• There is no warning when content exceeds 200 lines
• This limit is not mentioned in docs, --help, or the MEMORY.md file itself

Requested Changes

1. Document the limit — mention the 200-line cap in official docs
2. Add a warning — when MEMORY.md exceeds 200 lines, warn the user that content is being truncated
3. Make it configurable — allow users to set a custom line limit (e.g., in settings.json)

Why This Matters

Users relying on MEMORY.md as a persistent knowledge cache (which is its intended purpose) may unknowingly lose content beyond line 200. A user carefully curating important information at line 201+ would have no idea it's being silently dropped.

~Mark