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`.