feat(sandbox): scrub sensitive environment variables from package managers

Implements process-level environment variable protection per the spec.
When the sandbox is enabled, credential-bearing variables are removed from
the package manager child process before it is spawned, defending against
supply chain attacks that harvest secrets from the environment.

- DANGEROUS_ENV_VARS: curated default deny list of known secret names (no
  generic *_TOKEN/*_SECRET catch-alls); ScrubEnv matcher supports case-
  insensitive globs so profiles can opt into broader denies.
- EnvironmentPolicy (environment.allow / environment.deny) on sandbox
  profiles, merged under inheritance; deep-copied on resolve.
- npm/pypi profiles re-allow their own ecosystem's auth vars so package
  managers keep working; other ecosystems' and cloud creds stay scrubbed.
- New 'env' --sandbox-allow type (and overlay support via the same path):
  allow-only, value kept verbatim (not path-resolved), governed by lockdown.
- Enforced in executor.ApplySandbox as the last step before launch, after
  overlay and runtime overrides merge; scrubbed names logged for audit.

https://claude.ai/code/session_017Da1sAYLYpeEgogm6f9VYW

config/config.go

@@ -298,6 +298,7 @@ const (
 	SandboxAllowExec       SandboxAllowType = "exec"
 	SandboxAllowNetConnect SandboxAllowType = "net-connect"
 	SandboxAllowNetBind    SandboxAllowType = "net-bind"
+	SandboxAllowEnv        SandboxAllowType = "env"
 )
 
 // SandboxAllowOverride represents a single --sandbox-allow flag value.