chore: configure Codex worktree env setup

.codex/README.md

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+# Codex environment
+
+`environments/environment.toml` contains inline setup commands for macOS
+in `[setup]` and Windows PowerShell in `[setup.win32]`. Git must be on PATH.
+
+The setup copies the main Git checkout's root `.env` to the new worktree
+by locating its shared `.git` directory.
+It preserves an existing destination and skips copying if the source is missing.
+The file stays ignored by Git, and its contents are never logged.
+
+Commit the `.codex` directory so the setup is available in new worktrees.
+Keep a local `.env` in the main checkout on each machine.

.codex/environments/environment.toml

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+version = 1
+name = "Hydra"
+
+[setup]
+script = '''
+git_dir=$(git rev-parse --path-format=absolute --git-common-dir) || exit 1
+source_env="$(dirname "$git_dir")/.env"
+if [ ! -e .env ] && [ -f "$source_env" ]; then
+  cp -n "$source_env" .env
+fi
+'''
+
+[setup.win32]
+script = '''
+$ErrorActionPreference = 'Stop'
+$gitDir = git rev-parse --path-format=absolute --git-common-dir
+if ($LASTEXITCODE -ne 0) { throw 'Could not locate the main Git checkout.' }
+$sourceEnv = Join-Path (Split-Path -Parent $gitDir) '.env'
+if (!(Test-Path -LiteralPath '.env') -and (Test-Path -LiteralPath $sourceEnv -PathType Leaf)) {
+  [System.IO.File]::Copy($sourceEnv, (Join-Path (Get-Location).Path '.env'), $false)
+}
+'''