feat(cli): support Podman in `openshell doctor check`

[politerealism]

User Story

As an OpenShell user running the Podman compute driver, I want openshell doctor check to validate my Podman setup, so that I get the same actionable preflight diagnostics Docker users already get, instead of a check that always reports on Docker regardless of which driver I actually use.

Problem Statement

openshell doctor check (crates/openshell-cli/src/run.rs:215-251) unconditionally shells out to docker info to validate system prerequisites. It has no driver-detection logic and no Podman (or other compute driver) branch, so a user running the Podman driver gets a check that's irrelevant to their actual setup — it reports on Docker connectivity even when Docker isn't installed or in use, and it never validates the Podman socket, version, or rootless configuration the Podman driver actually depends on.

Impact / Why This Matters

Today, Podman users have no equivalent of doctor check to diagnose a broken Podman setup (missing socket, wrong Podman version, rootless misconfiguration) before running openshell. They're left with raw driver/gateway error messages or manual podman info inspection, instead of the single actionable pass/fail summary Docker users get. This is inconsistent with OpenShell's multi-driver design, where Podman is a fully supported, first-class compute driver, not a secondary one.

Proposed Design

openshell doctor check should detect which compute driver is configured (or accept an explicit override) and run the appropriate connectivity check: keep the existing docker info-based check for Docker, and add an equivalent check for Podman — verifying the Podman socket is reachable, reporting the Podman version, and validating rootless/rootful configuration analogous to what crates/openshell-driver-podman/src/driver.rs's own startup check already does inside the driver. The user-facing output format (labeled check line, "ok"/"FAILED" status, actionable guidance text) should stay consistent with the existing Docker check's style.

Acceptance Criteria

• openshell doctor check validates Podman connectivity when the configured/detected driver is Podman, with output in the same style as the existing Docker check.
• The check reports the Podman version on success, similar to Docker's --format {{.ServerVersion}}.
• The check fails with an actionable error (naming the relevant socket path or environment variable) when Podman is unreachable, mirroring the existing DOCKER_HOST-mentioning guidance for Docker.
• Existing Docker-only behavior is preserved for Docker users — this is additive, not a regression.

Alternatives Considered

Leave doctor check Docker-only and document it as such. Rejected because it leaves Podman users — a fully supported, first-class driver — without an equivalent preflight diagnostic tool, and the command's own framing ("system prerequisites," not "Docker prerequisites") implies broader driver coverage than it currently delivers.

Agent Investigation

Found while implementing #3663 (Podman CI coverage gaps). e2e/rust/tests/docker_preflight.rs tests exactly this Docker-only behavior end to end. doctor_check() in crates/openshell-cli/src/run.rs:215-251 unconditionally runs docker info --format {{.ServerVersion}} with no driver detection at all — confirmed by reading the function directly, not inferred. No Podman-equivalent CLI diagnostic exists. (A new e2e/rust/tests/podman_preflight.rs was added in PR #3690, but it tests a different thing: the Podman driver's behavior when its socket is unreachable, not the doctor check CLI command — it does not address this gap.)

Checklist

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