fix(shellenv): only export env vars Devbox adds or changes

[mikeland73]

Summary

Fixes #2826.

devbox shellenv and devbox global shellenv export the entire environment, including variables that Devbox never touches such as HOSTNAME and LANG. On distributions that mark some variables read-only (e.g. openSUSE marks PROFILEREAD read-only), eval "$(devbox global shellenv)" then fails with errors like:

(eval):55: read-only variable: PROFILEREAD

Re-exporting a variable to the value it already has is a no-op at best, and at worst breaks the user's prompt/shell on every command.

Fix

EnvExports now restricts the exported variables to those Devbox actually adds or changes relative to the current shell environment. Variables whose value already matches the ambient environment are dropped, since they are already present in the shell — so there is no reason to re-emit them (and no reason to trip over read-only ones).

• internal/devbox/envvars.go: add onlyModifiedEnvVars(env, ambient) helper that returns only new/changed variables.
• internal/devbox/devbox.go: in EnvExports, filter the computed env against os.Environ() when the new option is set. Skipped in pure mode, where the intent is to emit a complete, self-contained environment rather than a diff against the current shell.
• internal/devbox/devopt/devboxopts.go: add OnlyModifiedEnv to EnvExportsOpts.
• internal/boxcli/shellenv.go: enable OnlyModifiedEnv for the shellenv command (the reported path). The devbox shell --print-env / direnv path is intentionally left unchanged.

Variables Devbox relies on (PATH, the DEVBOX_* helpers, the shellenv hash, plugin/config env, etc.) are always new or changed, so they continue to be exported. Re-running shellenv inside an already-active environment now emits only what differs, which is idempotent.

Testing

• Added TestOnlyModifiedEnvVars in internal/devbox/envvars_test.go covering unchanged (dropped), changed (kept), and new (kept) variables, including a read-only-style PROFILEREAD case.
• go test ./internal/devbox/ -run 'TestOnlyModifiedEnvVars|TestExportify' passes.
• go build ./internal/..., go vet, and gofmt are clean.

(The remaining failures in ./internal/devbox/ unit tests are pre-existing and environmental — they require the nix binary, which isn't available in CI sandbox; they are unrelated to this change.)

cc @gasuketsu (issue reporter)

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