feat(cli): import externally issued OIDC tokens non-interactively

Problem Statement

Headless workloads can obtain a valid OIDC access token from an external identity broker, but OpenShell has no supported non-interactive command for importing that token into an existing OIDC gateway registration. gateway add --oidc-issuer and gateway login initiate login flows, so automation must either invoke an unsuitable interactive flow or write OpenShell's private token-storage format directly.

A supported import boundary would let CI systems and workload-identity brokers authenticate without exposing tokens on command lines or depending on an internal JSON schema.

Proposed Design

Add a non-interactive token-import command for an already registered OIDC gateway, for example:

openshell gateway token import --name <gateway> --file <token-bundle>

The command should:

• accept a bounded file or standard input, never require the access token as a command-line argument;
• require an existing gateway whose metadata declares OIDC authentication;
• accept an explicit, documented token-bundle schema containing the access token, absolute expiry, issuer, and client ID, with an optional refresh token;
• validate required metadata against the selected gateway registration and reject expired or near-expiry credentials;
• store credentials through openshell-bootstrap using its restricted-directory and owner-only file handling;
• avoid printing token values in normal output or errors;
• fail rather than opening a browser when invoked non-interactively; and
• leave refresh behavior explicit: use a supplied refresh token when supported, otherwise require the imported access token to cover the intended operation.

The storage schema should become a supported CLI input contract without making its on-disk path part of the public interface.

Alternatives Considered

• Write oidc_token.json directly. This works with current releases but depends on a private path and serialization format, duplicates permission handling, and can race other CLI invocations.
• Use browser login. This is appropriate for people but not for projected workload identity or unattended jobs.
• Add a client-credentials mode to every broker. That excludes brokers designed around projected assertions and still leaves token persistence to each integration.
• Pass a token through an environment variable. Environment inheritance and diagnostics make this easier to expose than a bounded file or standard input.

Agent Investigation

• Inspected OpenShell main at 736e431d and latest release v0.0.96.
• Reviewed GatewayCommands in crates/openshell-cli/src/main.rs: gateway registration supports OIDC metadata, while login remains a separate authentication flow and no token-import command exists.
• Reviewed crates/openshell-bootstrap/src/oidc_token.rs: OidcTokenBundle and safe storage helpers already provide the underlying persistence primitive.
• Reviewed current OIDC login and refresh code in crates/openshell-cli/src/oidc_auth.rs and gateway command handling.
• Searched open and closed issues for OIDC token import, non-interactive OIDC, and externally issued token support; no matching issue was found.

Checklist

• I've reviewed existing issues and the architecture docs
• This is a design proposal, not a "please build this" request

Start with GatewayCommands in crates/openshell-cli/src/main.rs, then read crates/openshell-bootstrap/src/oidc_token.rs and the OIDC flow in crates/openshell-cli/src/oidc_auth.rs. Define the command and token-bundle contract for an existing OIDC gateway, including validation and non-interactive behavior. Done means credentials are safely stored through openshell-bootstrap without exposing token values or relying on the private on-disk format.