docs: contributor guidelines still list `order` as a required config key

[estivate]

Need

Since 2.0.0, order: in a sync project's config.yml is optional: omitting it enables auto-tiered write ordering, and sync --parallel requires it to be absent. The contributor guidelines still present order as one of the expected config keys with no mention that it is optional, and automated review enforces the stale rule: CodeRabbit flagged the shipped example config on #147 for omitting order, citing the guideline. Every PR that touches an auto-tiered example config will receive the same false finding until the guidelines match the behavior.

Context

• AGENTS.md lines 83 and 85 list the YAML config keys as name, source, destination, order and state that order defines the sync sequence, with no mention of auto-tiering or optionality.
• .github/copilot-instructions.md repeats the same lines (AGENTS.md is the declared single source of truth; the platform files copy from it, so both need the update).
• The user-facing reference is already current: docs/docs/reference/config.mdx describes order as an override of the computed order. One adjacent spot, docs/docs/reference/schema-mapping.mdx line 96, still directs readers to "the order key" without noting the computed default.
• CodeRabbit reads the guidelines as path-specific coding rules, so correcting them also removes a recurring false positive from automated review.