Feature Request: Show context window consumption percentage

Problem

Automatic compaction interrupts workflow with no warning. There's no way to know how close you are to hitting the context window limit. When compaction triggers mid-task, it takes time and breaks concentration.

Request

Display context window usage as a percentage — in the status line, after each response, or via a /usage command — so users can proactively /clear or start a new session before compaction hits.

Use Case

When running multi-step workflows (invoking skills, executing test suites, iterating on file edits), context fills up unpredictably. Large tool outputs (curl responses, file reads, skill definitions) consume varying amounts of space. A visible indicator would let users make informed decisions about when to clear context vs. continue working.