feat: discourage the use of the --all-features flag (#12429)

## Why

Developers are frequently running low on disk space, and routine use of
`--all-features` contributes to larger Cargo build caches in `target/`
by compiling additional feature combinations.

This change updates local workflow guidance to avoid `--all-features` by
default and reserve it for cases where full feature coverage is
specifically needed.

## What Changed

- Updated `AGENTS.md` guidance for `codex-rs` to recommend `cargo test`
/ `just test` for full-suite local runs, and to call out the disk-usage
cost of routine `--all-features` usage.
- Updated the root `justfile` so `just fix` and `just clippy` no longer
pass `--all-features` by default.
- Updated `docs/install.md` to explicitly describe `cargo test
--all-features` as an optional heavier-weight run (more build time and
`target/` disk usage).

## Verification

- Confirmed the `justfile` parses and the recipes list successfully with
`just --list`.

justfile

@@ -28,10 +28,10 @@ fmt:
     cargo fmt -- --config imports_granularity=Item 2>/dev/null
 
 fix *args:
-    cargo clippy --fix --all-features --tests --allow-dirty "$@"
+    cargo clippy --fix --tests --allow-dirty "$@"
 
 clippy:
-    cargo clippy --all-features --tests "$@"
+    cargo clippy --tests "$@"
 
 install:
     rustup show active-toolchain
@@ -41,6 +41,8 @@ install:
 # --no-fail-fast is important to ensure all tests are run.
 #
 # Run `cargo install cargo-nextest` if you don't have it installed.
+# Prefer this for routine local runs; use explicit `cargo test --all-features`
+# only when you specifically need full feature coverage.
 test:
     cargo nextest run --no-fail-fast