feat: content size limits and warnings for large blobs

[flyingrobots]

Motivation

Nothing prevents a user from attaching a 100MB file to a node via content set. Since content is stored as git blobs, large attachments could bloat the repo. A warning or configurable limit would be good guardrails.

Proposed Behavior

• Warn on content set if file exceeds 1MB (configurable threshold)
• Hard reject above 10MB (configurable) unless --force is passed
• Show total content size in git mind doctor output

Implementation

• Check buf.length in writeContent / contentSet
• Add thresholds to graph config or extension manifest
• Add a content-size detector to doctor.js