Show progress when copying to Nix store (avoid appearing stuck)

[schickling]

Problem

When running devenv tasks run, the TUI can appear "stuck" for minutes with no visible progress. This typically happens when Nix is copying derivations to the store — an operation that can take a long time but provides no feedback to the user.

The progress TUI introduced in v1.8 significantly improved visibility into builds, downloads, and evaluations. However, store copy operations still appear as silent gaps where nothing seems to be happening.

Proposal

1. Show store copy progress in the TUI — Surface Nix store copy activity (e.g. "Copying X to store…") so users understand what's happening during otherwise silent periods.

2. Warn about excessive copies — If a large number of paths are being copied to the store (suggesting something may be misconfigured or inefficient), show a warning or summary. This would help users identify and fix unnecessarily large closures.

Context

• The nix_log_bridge already parses Nix's internal JSON logs — store copy events may be available or could be added
• Upstream Nix has known limitations with copy progress reporting (nix copy should show progress NixOS/nix#6533), but even basic "copying path X" visibility would be a big improvement
• Related: Idle gaps in building shell #2042 (idle gaps in building shell), Devenv builds and initializations are slow #1102 (slow initializations with no feedback)

[domenkozar]

cc @sandydoo

[sandydoo]

Yes, this is on the roadmap. This requires more involved Nix surgery as Nix itself doesn't track these copies very well.