fix: make flake dev env vars respect per-worktree overrides

[centdix]

Summary

Webmux/workmux worktrees get per-worktree DATABASE_URL, REMOTE, and port assignments via .env.local. However, the Nix flake's devEnvVars hardcoded these as derivation attributes, which direnv (use flake) applied unconditionally — overwriting the correct worktree-specific values with the main worktree defaults.

Changes

• Move DATABASE_URL, REMOTE, REMOTE_LSP from Nix attrs to a shellHook using ${VAR:-default} so they respect pre-existing values from webmux's runtime.env
• Add shellHook = devShellHook to both default and full devShells
• Update .envrc to source .env.local after use flake, so direnv picks up per-worktree overrides (needed because nix print-dev-env evaluates shellHook in a clean subshell)

Test plan

• Main worktree (no .env.local): echo $DATABASE_URL still shows windmill default
• Webmux worktree with .env.local: echo $DATABASE_URL shows worktree-specific DB, echo $REMOTE shows correct port
• nix develop without direnv: defaults still apply

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