Add --short flag or /ctx alias for /context command

Problem

/context outputs a full breakdown of every MCP tool, skill, memory file, and custom agent — often 100+ lines. The useful signal for most checks is just the top summary:

• Model name
• Total tokens used / limit
• Category breakdown table (system prompt, MCP tools, memory files, skills, messages, free space)

The detailed per-tool/per-skill tables are valuable for debugging but unnecessary for a quick "how much context am I using?" check.

Proposal

One of:

• /context --short — only show model + total tokens + category table
• /ctx — short alias that defaults to the summary view
• /context --full to opt IN to the detailed breakdown (flip the default)

Use case

Power users running /context frequently during sessions to monitor token budget. The detailed breakdown creates scroll fatigue when all you need is the 10-line summary.

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