docs(AGENTS.md): make just the only build/test surface for agents - #1115

The platform matrix read as "macOS cannot build or test minimald", which
is only true of a *native* build: `just test-cross` cross-compiles the
whole workspace (minus minvmd) to musl in Docker and runs its tests, so
the daemon and the CLI's test targets are reachable from a Mac. Agents
took the old wording as licence to skip that coverage, or to hand-roll
cargo invocations around it.

- State the native-only limit, then name `just test-cross` as the macOS
  answer for everything the darwin scope skips, and say plainly that a
  minimald / CLI-test change is unverified until it has run.
- Add the just-only rule: the recipes carry the per-OS scoping, the
  pinned flags, and the ordering constraints (the macOS codesign, most
  sharply); a hand-rolled cargo line drops them silently. If a task has
  no recipe, add one.
- Replace the two remaining raw-cargo instructions — the `cargo test -p
  <crate>` inner loop and the `cargo clippy --allow-dirty --fix` variant
  — with `just test` and `just fix`, which are those commands, scoped.
- Stop quoting runnable cargo lines in the recipe descriptions; they read
  as commands to copy.

Refresh the recipe inventory while here: the count (33) and the list had
both fallen nine recipes behind the justfile. Now 42 on Linux / 37 on
macOS, with fix, lint-shell, msrv, miri, fuzz, fuzz-check,
test-promote-gate, stress, bulk-upload, and kernel-review documented.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

…ventory

Per review: the recipe count and the four enumerated recipe lists were both
going to go stale, and the lists gobble context for no gain — `just --list`
already prints every recipe available on the host, each with its own
description, generated from the justfile.

- Drop the "42 on Linux / 37 on macOS" count. It moots the arithmetic
  CodeRabbit flagged (five vs six) by removing the claim rather than
  correcting a number that changes every time a recipe lands.
- Drop the four enumerated lists (CI-parity gates, e2e & harnesses, stack
  bring-up, build artifacts): 53 lines that restate `just --list`.
- Keep what the generated list cannot carry: the justfile/CI-twin
  relationship, the `.scratch/` scoping rule, and the `just soak` warning.

That last one is kept deliberately. `just --list` renders only the *last*
comment line above a recipe, so soak's leading
"# Reaps between iterations — this WILL kill this checkout's live dev stack"
never reaches the listing; only the trailing description does.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>