fix(network): sg show/delete accept name as well as ID (closes #194)

[crowdy]

Summary

• `conoha network sg delete ` returned an HTTP 405 from the upstream OpenStack endpoint because the CLI passed the literal name where the API expects a UUID. Other commands (e.g., `server delete <name|id>`) already accept either; this aligns sg show/delete with that.
• New `NetworkAPI.FindSecurityGroup(nameOrID)` helper, used by both `sg show` and `sg delete`. The existing internal `resolveSecurityGroupID` now delegates to it.
• `sg show` re-fetches by ID after the lookup so the rules array is included (list endpoint can return abbreviated form).
• The `sg delete` confirm prompt now shows both name and ID, and accepts an exact name match (no fuzzy resolution).

Test plan

• `go test ./internal/api/... -run Find` — new `TestSecurityGroupFindSecurityGroup` covers by-name, by-id, not-found
• `go test ./...` — full suite green
• Live verify: `conoha network sg create --name temp-sg && conoha network sg delete temp-sg` succeeds (no 405)
• Live verify: `conoha network sg show temp-sg` returns details

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