refactor(config): separate source loading from resolution

[natalie-o-perret]

Follow-up to #817.

Splits config source loading from resolution into a small, testable layer.

Each source (env vars, config file profile, built-in defaults) is loaded independently into its own struct, then merged by resolve() with one explicit precedence rule:

CLI flags > env vars > config file profile > built-in defaults

Changes:

• New cmd/config_resolution.go with envSources, fileSources, and resolve(). Replaces envAccountOverrides, useEnvOnlyAccount, and finalizeCurrentAccount.
• Fix --config vs EXOSCALE_CONFIG precedence: CLI flags now always win. Previously EXOSCALE_CONFIG silently overrode --config because the env-to-flag mapping had no Changed guard.
• Add EXOSCALE_ZONE support: overrides the default zone from the selected config profile without touching anything else.
• 11 unit tests covering every precedence rule in resolve().
• 2 new e2e testscript scenarios for EXOSCALE_ZONE and --config / EXOSCALE_CONFIG precedence.

Testing:

go test ./cmd/...
make build
cd tests/e2e && go test -run TestScriptsLocal -count=1 .