docs(skills): document context/background; opt Knowledge out of 2.1.218 background default

[elhoim]

Problem

Two related gaps:

1. SkillSystem.md documents the skill frontmatter contract but never mentions context: or background: — despite seven shipped skills declaring context: fork.
2. skills/Knowledge/SKILL.md declares context: fork with no background value, so it inherits a default that changed underneath it.

Why it matters

Claude Code 2.1.218 changed skills declaring context: fork to run as background subagents by default. Previously they blocked the main thread.

A backgrounded skill returns via task notification rather than inline output. For long-running deliberation skills that is an improvement — detaching them is the point. But Knowledge is an interactive lookup skill (effort: low, argument-hint [search|...|retrieve|<query>]). A user asking "what do we know about X" expects the answer in that turn. Backgrounded, it doesn't arrive there.

Because neither field was documented, there was no way to discover the opt-out short of reading the changelog.

Fix

• Documents context and background in SkillSystem.md with a field table and the 2.1.218 gotcha, plus the test for which way to set it.
• Adds background: false to Knowledge only.

The other six fork skills — Council, Evals, Ideate, Research, RootCauseAnalysis, SystemsThinking — are all effort: high deliberation and are deliberately left on the new background default.

Affected versions

Introduced in Claude Code 2.1.218. background is a real parsed frontmatter field in that build, verified against the shipped binary.

[danielmiessler]

The Knowledge background: false half had already landed in source independently — you called the right skill. The documentation half is now ported: SkillSystem.md documents context and background with a field table and the 2.1.218 default-change gotcha, including your test for which way to set it. Credited; ships in the next release.