[FEATURE] /context should default to graph only; /context all for full output

This has been filed at least four times. All previously closed without resolution:

• #18056 - /context --brief (closed)
• #19595 - /context -m flag (closed)
• #27592 - /context --compact (closed)
• #48798 - /context is too verbose (closed)

The problem has not been addressed and previous issues have been ignored or autoclosed.

Problem

The /context command exists to answer one question: how full is my context window?

The current output buries that answer under expanded listings of MCP tools, custom
agents, memory files, and skills - none of which the user asked for. Worse, that
output adds thousands of tokens to the session, so running /context to monitor
token usage actively makes the problem worse.

Proposal

Split into two modes:

/context - shows only the usage graph and category summary:

Context Usage
⛀ ⛁ ⛁ ⛁ ⛁ ⛁ ⛀ ⛁ ⛀ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ Opus 4.8 (1M context)
⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ claude-opus-4-8[1m]
⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ 35.8k/1m tokens (4%)
...
⛁ System prompt: 2.8k tokens (0.3%)
⛁ System tools: 20.2k tokens (2.0%)
⛁ Custom agents: 6.1k tokens (0.6%)
⛁ Memory files: 2.6k tokens (0.3%)
⛁ Skills: 4k tokens (0.4%)
⛁ Messages: 70 tokens (0.0%)
⛶ Free space: 964.2k (96.4%)

/context all - the current full output including expanded MCP tools,
agent listings, memory files, and skills. Those are also individually accessible
via /mcp, /agents, /memory, and /skills.

This is a low-risk change: the current behavior simply becomes /context all,
and the default gets tightened to what users actually need.

Why this keeps getting filed

Each prior issue was closed by automation - marked duplicate or stale - before
the feature shipped. The demand is real and consistent across all five filings.

Users do care about context usage - that is exactly why they run /context. But
what they need in the moment is a quick, high-level read: how full is the window
and where is the pressure coming from. The current command makes that harder, not
easier, by burying the answer in a wall of detail.