* shell: add s3.user.* and s3.policy.attach|detach commands
Add focused IAM shell commands following a noun-verb model:
- s3.user.create: create user with auto-generated or explicit credentials
- s3.user.list: tabular listing with status, policies, key count
- s3.user.show: detailed user view (status, source, policies, credentials)
- s3.user.delete: delete a user
- s3.user.enable: enable a disabled user
- s3.user.disable: disable a user (preserves credentials and policies)
- s3.policy.attach: attach a named policy to a user
- s3.policy.detach: detach a policy from a user
These commands are thin wrappers over the existing IAM gRPC service,
producing human-readable output instead of raw protobuf text.
This is part of a larger effort to replace the monolithic s3.configure
command with a composable set of single-purpose commands.
* shell: address review feedback for s3.user.* and s3.policy.attach|detach
- Return flag parse errors instead of swallowing them (all commands)
- Use GetConfiguration instead of N+1 GetUser calls in s3.user.list
- Add nil check for resp.Identity in s3.user.show
- Fix GetPolicy error masking in s3.policy.attach (wrap original error)
- Simplify joinMax using strings.Join
* shell: add nil identity guards and wrap gRPC errors
- Add nil check for resp.Identity in policy_attach, policy_detach,
user_enable, user_disable
- Wrap GetUser errors with user context for better diagnostics
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