[CAS-002] CLI content set|show|meta|delete commands

[flyingrobots]

CAS-002: CLI content commands

Parent: #271 (M13 VESSEL)
Depends: #272 (CAS-001)

Deliverable

Four new CLI commands in src/cli/commands.js:

• git mind content set <node> --from <file> — read file, detect MIME, write to CAS, set properties
• git mind content show <node> [--raw] — read and display content (--raw for piping, no metadata header)
• git mind content meta <node> [--json] — show content metadata
• git mind content delete <node> — remove content attachment

Design

• MIME detection: use file extension mapping (.md → text/markdown, .txt → text/plain, etc.)
• content set accepts --mime override for explicit MIME type
• content show without --raw prints a metadata header then content body
• content delete confirms node exists and has content before removing
• --json support on all commands following established pattern
• Chalk formatting for human-readable output