docs: describe the crate AGENTS.md now compiles

[LNSD]

AGENTS.md and the skills still described a feature axis with one switch on it, so an agent reading them would gate new code on a feature that no longer exists and prove its no_std claim with a command that no longer builds the layer the claim is about.

• Replace the filesystem-support feature table with the two axes that select a build now: which formats are compiled, and whether the runtime carries an allocator or the standard library.
• Correct the no_std proof to the command CI runs, and say why it takes alloc with the formats: write is where a use std:: would otherwise creep back in unnoticed.
• Point at the ## doc comments in Cargo.toml as the authoritative description of each feature, so the table has one place to drift from rather than being the record itself.
• Note check-unused-deps alongside the other standalone CI jobs, and show a single-format build among the examples.
• Point both of the check skill's no-std invocations at the same flags, restate the signals that call for that configuration in terms of a std gate, and add src/write/ to them, since the builders are now compiled without std and can break there too.
• Correct the test and review skills, which described the suite and the compatibility rule by the same removed feature.