refactor: replace manual env overrides with caarlos0/env

[deevus]

Summary

• Replace 15 manual applyEnv* calls and 4 helper functions with env.Parse(cfg) from caarlos0/env/v11
• Add explicit env:"PIXELS_*" struct tags to all config fields with env var overrides
• Add TestEmptyEnvDoesNotOverrideDefault to guard against future library behavior changes
• Net -24 lines, zero transitive dependencies added

Details

The config loading had ~50 lines of boilerplate wiring each PIXELS_* env var to its config field via type-specific helpers (applyEnv, applyEnvInt, applyEnvInt64, applyEnvBool). caarlos0/env replaces all of it with struct tags and a single function call.

Chosen over spf13/viper (heavy deps, key-lowercasing friction) and knadh/koanf (non-mechanical env var naming makes TransformFunc awkward).

Preserved behavior:

• Config priority: TOML defaults < env vars < CLI flags
• *bool nil-when-unset semantics (pointer stays nil when env var absent)
• TOML loading, expandHome, os.ExpandEnv post-processing unchanged
• Error handling improved: invalid env values now surface errors instead of being silently ignored

Test plan

• go build compiles
• go test ./... all 11 config tests pass (including env override, provision bool, empty env)
• go vet ./... clean

[github-actions]

⬇️ Go test coverage decreased from 28.5% to 21.5% compared to 206fe28

⚠️ 1 of 8 packages have zero coverage.

• github.com/deevus/pixels

Updated Package Coverages:

# Package Name                             |  Prior |    New
- github.com/deevus/pixels/cmd             |   6.8% |   5.3%
- github.com/deevus/pixels/internal/cache  |  93.8% |  83.3%
- github.com/deevus/pixels/internal/config |  88.9% |  87.1%
- github.com/deevus/pixels/internal/ssh    |  29.3% |  14.7%

View coverage for all packages

# Package Name                              | Coverage
- github.com/deevus/pixels                  |     0.0%
+ github.com/deevus/pixels/cmd              |     5.3%
+ github.com/deevus/pixels/internal/cache   |    83.3%
+ github.com/deevus/pixels/internal/config  |    87.1%
+ github.com/deevus/pixels/internal/dataset |   100.0%
+ github.com/deevus/pixels/internal/egress  |    93.5%
+ github.com/deevus/pixels/internal/ssh     |    14.7%
+ github.com/deevus/pixels/internal/truenas |    68.5%