feat(cli): warn when --env values look like credentials

[letv1nnn]

Summary

openshell sandbox create now emits a non-blocking warning when an --env key looks like a credential, steering users toward providers so secrets aren't exposed to the sandboxed agent. The warning suggests a specific provider create command when the key matches a built-in provider profile, and otherwise links to the providers docs.

Related Issue

#2548

Changes

• Add credential_env_matches in crates/openshell-cli/src/commands/common.rs: flags --env keys by name only — known provider env vars from builtin_profiles(), plus *_TOKEN, *_SECRET, *_PASSWORD, *_CREDENTIAL, *_ACCESS_KEY, *_SECRET_KEY, *_API_KEY suffixes (case-insensitive). Never inspects or prints values.
• Add warn_credential_env_vars: prints a non-blocking stderr warning per flagged key. When built-in profiles claim the key it lists a specific openshell provider create command per match (a key can map to several, e.g. GITHUB_TOKEN → copilot and github); otherwise it links to the providers docs.
• Add the --no-credential-warnings flag to sandbox create and wire the warning in before the CreateSandbox RPC (create only, not exec).
• Re-export warn_credential_env_vars through run to match the existing parse_env_pairs call pattern.
• Unit tests in common.rs covering profile match, suffix-only match, case-insensitivity, non-credential skip,
  no-value-leak, and deterministic ordering.
• Document the warning and flag in docs/sandboxes/manage-sandboxes.mdx.

Example output

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Publisher/user docs updated (if applicable)
• Architecture docs updated (if applicable)

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[mrunalp]

instead of suffixes could check for presence anywhere within the key.