packaging: disambiguate the wit-wiring extensions.toml from the engine.toml extensions convention

[mfw78]

Follow-up from review of nullislabs/shepherd#440 (comment). Confirmed still valid at the end of the completed shepherd feature train.

Problem

The root extensions.toml (WIT wiring: [extensions.client] carrying import / packages, loaded at macro-expansion time via the ancestor walk) uses the same [extensions.<name>] section heading as the engine.toml runtime operator config documented in docs/design/linker-extension-seam.md (for example [extensions.cow] with orderbook_urls, parsed at runtime into an opaque toml::Value). Same section syntax, two unrelated files, different schemas, different load times, and nothing cross-referencing them. A reader who knows the engine.toml convention has no signal these are not the same mechanism.

Confirmed still present at HEAD: extensions.toml still carries [extensions.client], and the seam doc still reserves [extensions.<name>] for the runtime config.

Change

Either rename the WIT-wiring file to a distinct name (for example wit-extensions.toml or capabilities.toml), or add a cross-reference in both places (the file header and linker-extension-seam.md) noting the two are unrelated.