Tracked .envrc from #26005 silently overwrites contributors' local direnv config #26101

Problem

#26005 added a tracked .envrc (use flake). Before that change .envrc was
not tracked, so contributors who use direnv kept their own local .envrc
(e.g. CARGO_TARGET_DIR, CARGO_INCREMENTAL, helper exports). Since
.envrc is a common entry in global gitignores and .git/info/exclude, many
of these files are ignored rather than untracked.

Git treats ignored files as expendable, so updating to a commit that tracks
.envrc replaces the local file without any warning:

echo 'export CARGO_TARGET_DIR=/tmp/df-target' > .envrc
echo .envrc >> .git/info/exclude
git switch main            # any commit including #26005
cat .envrc                 # -> "use flake"; local config is gone

Contributors whose .envrc is untracked but not ignored instead get
error: The following untracked working tree files would be overwritten by checkout: .envrc and have to move the file aside by hand.

Why it matters

• Silent loss of local developer config on git pull / git switch.
• No supported place for per-contributor direnv settings: editing the tracked
  .envrc leaves a dirty tree that blocks branch switches and is easy to
  commit by accident in a PR.
• Contributors who do not use Nix but do use direnv now get a use flake
.envrc they cannot use as-is.

Invariant / desired behavior

• Nix users still get the dev shell from the tracked .envrc with no extra
  steps.
• Contributors can keep personal direnv settings in an ignored file that git
  never overwrites, and that is loaded automatically.

Proposed direction

Use the standard direnv local-override pattern:

# .envrc
use flake
source_env_if_exists .envrc.local

# .gitignore
.envrc.local

Optionally add a short note to the
development environment guide
describing the Nix/direnv setup and .envrc.local for personal settings.

Scope

In

• Add source_env_if_exists .envrc.local to .envrc.
• Add .envrc.local to .gitignore.
• Brief contributor-guide note (optional).

Out

• Changes to flake.nix / flake.lock.
• Removing the tracked .envrc or the Nix support added in feat: add Nix flake and direnv support #26005.

Acceptance criteria

• With Nix + direnv, entering the repo still loads the flake dev shell.
• A .envrc.local in the repo root is sourced by direnv after use flake.
• git status does not show .envrc.local, and git switch /
  git pull never modify it.

Tests / verification

• Manual: create .envrc.local with export FOO=1, run direnv allow,
  confirm echo $FOO prints 1 and git status is clean.
• Manual: switch between commits before and after the change; confirm
  .envrc.local is untouched.